diff --git a/.gitignore b/.gitignore
index f5f7e14c..06c75db6 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,6 +7,7 @@
 
 !srcs
 !srcs/Client
+!srcs/Debug
 !srcs/Result
 !srcs/Server
 !srcs/Socket
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 95d99c53..9b47a440 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3,7 +3,9 @@ project(webserv)
 
 set(CMAKE_CXX_STANDARD 98)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
-set(CMAKE_CXX_FLAGS "-Wall -Wextra -Werror -pedantic -g -fsanitize=address,undefined -fno-omit-frame-pointer")
+set(CMAKE_CXX_FLAGS "-Wall -Wextra -Werror -pedantic")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -g -fsanitize=address,undefined -fno-omit-frame-pointer")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D DEBUG")
 
 # google test ------------------------------------------------------------------
 include(FetchContent)
@@ -25,6 +27,7 @@ enable_testing()
 include_directories(
         includes
         srcs/Client
+        srcs/Debug
         srcs/Server
         srcs/Socket
 )
@@ -33,6 +36,7 @@ include_directories(
 set(webserv_srcs
         srcs/get_valid_config_file_path.cpp
         srcs/Client/Client.cpp
+        srcs/Debug/Debug.cpp
         srcs/Server/Server.cpp
         srcs/Socket/Socket.cpp
 )
diff --git a/Makefile b/Makefile
index 4ff410ef..8141e14d 100644
--- a/Makefile
+++ b/Makefile
@@ -20,6 +20,9 @@ SRCS		+=	$(SERVER_DIR)/Server.cpp
 SOCKET_DIR	=	Socket
 SRCS		+=	$(SOCKET_DIR)/Socket.cpp
 
+#debug
+DEBUG_DIR	=	Debug
+SRCS		+=	$(DEBUG_DIR)/Debug.cpp
 
 # OBJS -------------------------------------------------------------------------
 OBJS_DIR	=	objs
@@ -32,6 +35,7 @@ DEPS		=	$(OBJS:%.o=%.d)
 
 # INCLUDES ---------------------------------------------------------------------
 INCLUDES_DIR =	includes \
+				$(SRCS_DIR)/$(DEBUG_DIR) \
 				$(SRCS_DIR)/$(SERVER_DIR) \
 				$(SRCS_DIR)/$(SOCKET_DIR)
 INCLUDES	 =	$(addprefix -I, $(INCLUDES_DIR))
diff --git a/srcs/Client/Client.cpp b/srcs/Client/Client.cpp
index b136ecef..83606b15 100644
--- a/srcs/Client/Client.cpp
+++ b/srcs/Client/Client.cpp
@@ -1,5 +1,6 @@
 #include <arpa/inet.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <sys/socket.h>
 #include <unistd.h>
 #include <cstdio>
@@ -7,9 +8,9 @@
 #include <cstring>
 #include <iostream>
 #include <stdexcept>
-#include "Client.hpp"
 #include "webserv.hpp"
 #include "Color.hpp"
+#include "Client.hpp"
 
 Client::Client(const char *server_ip, const char *server_port) {
 	this->_connect_fd = create_connect_socket();
@@ -49,15 +50,9 @@ struct sockaddr_in Client::create_connect_addr(const char *server_ip,
 }
 
 void Client::process_server_connect(const std::string &send_msg) {
-	// printf(MAGENTA "client(pid:%d) 1 start\n" RESET, getpid());
-	// printf(MAGENTA "client(pid:%d) 2 connect\n" RESET, getpid());
 	connect_to_server(this->_connect_fd, this->_addr);
-	// printf(MAGENTA "client(pid:%d) 3 send\n" RESET, getpid());
 	send_to_server(this->_connect_fd, send_msg);
-	// printf(MAGENTA "client(pid:%d) 4 recv\n" RESET, getpid());
 	this->_recv_message = recv_message_from_server(this->_connect_fd);
-	// printf(MAGENTA " client(pid:%d) recv_msg:[%s]\n" RESET, getpid(), this->_recv_message.c_str());
-	// printf(MAGENTA "client(pid:%d) 5 end\n" RESET, getpid());
 }
 
 void Client::connect_to_server(int connect_fd, struct sockaddr_in addr) {
@@ -72,7 +67,6 @@ void Client::connect_to_server(int connect_fd, struct sockaddr_in addr) {
 
 void Client::send_to_server(int connect_fd, const std::string &send_msg) {
 	ssize_t	send_size;
-	// std::cout << "msg:" << send_msg << std::endl;
 	size_t	msg_len = send_msg.size() + 1;
 	const char *msg = send_msg.c_str();
 
@@ -89,28 +83,19 @@ std::string Client::recv_message_from_server(int connect_fd) {
 	char		buf[BUFSIZ + 1];
 	std::string	recv_msg;
 
-	// printf(MAGENTA " client(pid:%d) 4-1 recv_msg start\n" RESET, getpid());
-
 	while (true) {
-		// printf(MAGENTA " client(pid:%d) 4-2 recv_msg recv\n" RESET, getpid());
 		errno = 0;
 		recv_size = recv(connect_fd, &buf, BUFSIZ, 0);
 		if (recv_size == ERROR) {
-			// printf(MAGENTA "  client(pid:%d) recv_err errno:%d\n" RESET, getpid(), errno);
 			std::string err_str = "[Client Error] recv: " + std::string(strerror(errno));
 			throw std::runtime_error(RED + err_str + RESET);
 		}
 		buf[recv_size] = '\0';
-		// printf(MAGENTA " client(pid:%d) 4-3 buf:[%s]\n" RESET, getpid(), buf);
 		recv_msg += buf;
 		if (recv_size < BUFSIZ) {
-			// std::cout << MAGENTA " 3-6" RESET << std::endl;
 			break;
 		}
-		// printf(MAGENTA " client(pid:%d) 4-4 next\n" RESET, getpid());
 	}
-	// printf(MAGENTA " client(pid:%d) 4-5 recv_msg fin\n" RESET, getpid());
-
 	return recv_msg;
 }
 
diff --git a/srcs/Debug/Debug.cpp b/srcs/Debug/Debug.cpp
new file mode 100644
index 00000000..a36ce769
--- /dev/null
+++ b/srcs/Debug/Debug.cpp
@@ -0,0 +1,37 @@
+# include <unistd.h>
+# include <stdarg.h>
+# include <cstdio>
+# include "Color.hpp"
+# include "Debug.hpp"
+
+pthread_mutex_t print_mutex = PTHREAD_MUTEX_INITIALIZER;
+
+void DEBUG_SERVER_PRINT(const char *fmt, ...) {
+#ifdef DEBUG
+	pthread_mutex_lock(&print_mutex);
+	dprintf(STDERR_FILENO, "%s%s", BLUE, "#DEBUG server: ");
+	va_list args;
+	va_start(args, fmt);
+	vdprintf(STDERR_FILENO, fmt, args);
+	va_end(args);
+	dprintf(STDERR_FILENO, "%s\n", RESET);
+	pthread_mutex_unlock(&print_mutex);
+#else
+	(void)fmt;
+#endif
+}
+
+void DEBUG_CLIENT_PRINT(const char *fmt, ...) {
+#ifdef DEBUG
+	pthread_mutex_lock(&print_mutex);
+	dprintf(STDERR_FILENO, "%s%s", MAGENTA, "#DEBUG client: ");
+	va_list args;
+	va_start(args, fmt);
+	vdprintf(STDERR_FILENO, fmt, args);
+	va_end(args);
+	dprintf(STDERR_FILENO, "%s\n", RESET);
+	pthread_mutex_unlock(&print_mutex);
+#else
+	(void)fmt;
+#endif
+}
diff --git a/srcs/Debug/Debug.hpp b/srcs/Debug/Debug.hpp
new file mode 100644
index 00000000..92e05631
--- /dev/null
+++ b/srcs/Debug/Debug.hpp
@@ -0,0 +1,8 @@
+#pragma once
+
+# include <pthread.h>
+
+extern pthread_mutex_t print_mutex;
+
+void DEBUG_SERVER_PRINT(const char *fmt, ...);
+void DEBUG_CLIENT_PRINT(const char *fmt, ...);
diff --git a/srcs/Server/Server.cpp b/srcs/Server/Server.cpp
index 791b891e..a34bb8aa 100644
--- a/srcs/Server/Server.cpp
+++ b/srcs/Server/Server.cpp
@@ -10,6 +10,7 @@
 #include <vector>
 #include "webserv.hpp"
 #include "Color.hpp"
+#include "Debug.hpp"
 #include "Result.hpp"
 #include "Server.hpp"
 
@@ -133,6 +134,7 @@ Result<int, std::string> Server::communicate_with_ready_client() {
 		}
 		this->_recv_message = recv_result.get_ok_value();
 		// printf(BLUE "   server recv_msg:[%s]\n" RESET, this->_recv_message.c_str());
+		DEBUG_SERVER_PRINT("connected. recv:[%s]", this->_recv_message.c_str());
 
 		// request, response
 		HttpRequest request = HttpRequest(this->_recv_message);
diff --git a/srcs/Server/Server.hpp b/srcs/Server/Server.hpp
index c97803b1..71078bb8 100644
--- a/srcs/Server/Server.hpp
+++ b/srcs/Server/Server.hpp
@@ -40,7 +40,7 @@ class Server {
 
  private:
 	Socket _socket;
-	std::string _recv_message;
+	std::string _recv_message;  // for test. this variable valid only connect with 1 client
 	std::vector<int> _client_fds;
 	fd_set _fds;
 
diff --git a/test/unit_test/TestServer.cpp b/test/unit_test/TestServer.cpp
index a6dfb7e6..00a12d41 100644
--- a/test/unit_test/TestServer.cpp
+++ b/test/unit_test/TestServer.cpp
@@ -1,7 +1,5 @@
 #include <netdb.h>
-#include <sys/socket.h>
 #include <pthread.h>
-#include <cstdio>
 #include <exception>
 #include <string>
 #include <vector>
@@ -9,19 +7,21 @@
 #include "webserv.hpp"
 #include "Client.hpp"
 #include "Color.hpp"
+#include "Debug.hpp"
 #include "Server.hpp"
 
 struct s_server {
-	const char *server_ip;
-	const char *server_port;
-	std::string recv_msg;
+	const char	*server_ip;
+	const char	*server_port;
+	std::string	recv_msg;
 };
 
 struct s_client {
-	const char *server_ip;
-	const char *server_port;
-	std::string send_msg;
-	std::string recv_msg;
+	int			no;
+	const char	*server_ip;
+	const char	*server_port;
+	std::string	send_msg;
+	std::string	recv_msg;
 };
 
 static void run_server_and_client(const char *server_ip,
@@ -259,9 +259,13 @@ static void *run_server(void *server_info) {
 	bool		is_server_success = true;
 
 	try {
+		DEBUG_SERVER_PRINT("start");
 		Server server = Server(s->server_ip, s->server_port);
+		DEBUG_SERVER_PRINT("connecting...");
 		server.process_client_connection();
 		s->recv_msg = server.get_recv_message();
+		// vvv this func can print only 1 message vv
+		// printf("server connected. recv:[%s]\n", s->recv_msg.c_str());
 	}
 	catch (std::exception const &e) {
 		is_server_success = false;
@@ -275,10 +279,13 @@ static void *run_client(void *client_info) {
 	bool		is_client_success = true;
 
 	try {
+		DEBUG_CLIENT_PRINT("no:%d start", c->no);
 		Client client = Client(c->server_ip, c->server_port);
 		sleep(1);
+		DEBUG_CLIENT_PRINT("no:%d connecting...", c->no);
 		client.process_server_connect(c->send_msg);
 		c->recv_msg = client.get_recv_message();
+		DEBUG_CLIENT_PRINT("no:%d connected. recv:[%s]", c->no, c->recv_msg.c_str());
 	}
 	catch (std::exception const &e) {
 		is_client_success = false;
@@ -293,7 +300,7 @@ static void run_server_and_client(const char *server_ip,
 								  std::string &server_recv_msg,
 								  std::string &client_recv_msg) {
 	s_server server_info = {server_ip, server_port, ""};
-	s_client client_info = {server_ip, server_port, client_send_msg, ""};
+	s_client client_info = {0, server_ip, server_port, client_send_msg, ""};
 	pthread_t server_tid, client_tid;
 	int ret_server, ret_client;
 	bool is_server_success, is_client_success;
@@ -315,12 +322,21 @@ static void run_server_and_client(const char *server_ip,
 	if (!is_client_success) {
 		throw std::runtime_error("client error");
 	}
-	// std::cout << "server_recv_msg:" << server_info.recv_msg << std::endl;
-	// std::cout << "client_recv_msg:" << client_info.recv_msg << std::endl;
 	server_recv_msg = server_info.recv_msg;
 	client_recv_msg = client_info.recv_msg;
 }
 
+static std::vector<s_client> init_client_infos(int client_count,
+											   const char *server_ip,
+											   const char *server_port,
+											   const std::string &client_send_msg) {
+	std::vector<s_client> client_infos(client_count, {0, server_ip, server_port, client_send_msg, ""});
+	for (int i = 0; i < client_count; ++i) {
+		client_infos[i].no = i;
+	}
+	return client_infos;
+}
+
 static void run_server_and_multi_client(const char *server_ip,
 										const char *server_port,
 										const std::string &client_send_msg,
@@ -328,7 +344,7 @@ static void run_server_and_multi_client(const char *server_ip,
 										std::vector<std::string> &client_recv_msgs,
 										int client_count) {
 	s_server server_info = {server_ip, server_port, ""};
-	std::vector<s_client> client_infos(client_count, {server_ip, server_port, client_send_msg, ""});
+	std::vector<s_client> client_infos = init_client_infos(client_count, server_ip, server_port, client_send_msg);
 	pthread_t server_tid;
 	std::vector<pthread_t> client_tids(client_count);
 	int ret_server, ret_client;
@@ -355,8 +371,6 @@ static void run_server_and_multi_client(const char *server_ip,
 			throw std::runtime_error("client error");
 		}
 	}
-	// std::cout << "server_recv_msg:" << server_info.recv_msg << std::endl;
-	// std::cout << "client_recv_msg:" << client_info.recv_msg << std::endl;
 	server_recv_msg = server_info.recv_msg;
 	for (int i = 0; i < client_count; ++i) {
 		client_recv_msgs[i] = client_infos[i].recv_msg;