diff --git a/src/tateyama/proto/session/request.proto b/src/tateyama/proto/session/request.proto
index f96cf3e..02ac1a7 100644
--- a/src/tateyama/proto/session/request.proto
+++ b/src/tateyama/proto/session/request.proto
@@ -2,6 +2,10 @@ syntax = "proto3";
 
 package tateyama.proto.session.request;
 
+option java_multiple_files = false;
+option java_package = "com.tsurugidb.session.proto";
+option java_outer_classname = "SessionRequest";
+
 // the request message to session pseudo service.
 message Request {
     // service message version (major)
@@ -82,8 +86,11 @@ message SessionSetVariable {
     // the target variable name, case insensitive.
     string name = 2;
 
-    // the text represented value to set.
-    string value = 3;
+    // the value to set, or NOT_SET to unset the variable.
+    oneof value_opt {
+        // the text represented value to set.
+        string value = 3;
+    }
 }
 
 
diff --git a/src/tateyama/session/session.cpp b/src/tateyama/session/session.cpp
index 387610e..daf042c 100644
--- a/src/tateyama/session/session.cpp
+++ b/src/tateyama/session/session.cpp
@@ -324,7 +324,9 @@ tgctl::return_code session_swtch(std::string_view session_ref, std::string_view
         auto* command = request.mutable_session_set_variable();
         command->set_session_specifier(std::string(session_ref));
         command->set_name(std::string(set_key));
-        command->set_value(std::string(set_value));
+        if (!set_value.empty()) {
+            command->set_value(std::string(set_value));
+        }
         auto response_opt = transport->send<::tateyama::proto::session::response::SessionSetVariable>(request);
         request.clear_session_set_variable();
 
diff --git a/src/tateyama/session/session.h b/src/tateyama/session/session.h
index a077d2e..ef49daf 100644
--- a/src/tateyama/session/session.h
+++ b/src/tateyama/session/session.h
@@ -26,6 +26,6 @@ namespace tateyama::session {
     tgctl::return_code session_list();
     tgctl::return_code session_show(std::string_view session_ref);
     tgctl::return_code session_shutdown(std::string_view session_ref);
-    tgctl::return_code session_swtch(std::string_view session_ref, std::string_view set_key, std::string_view set_value);
+    tgctl::return_code session_swtch(std::string_view session_ref, std::string_view set_key, std::string_view set_value = "");
 
 } //  tateyama::session
diff --git a/src/tateyama/tgctl/tgctl.cpp b/src/tateyama/tgctl/tgctl.cpp
index 70410a4..b6fe7d2 100644
--- a/src/tateyama/tgctl/tgctl.cpp
+++ b/src/tateyama/tgctl/tgctl.cpp
@@ -182,10 +182,13 @@ int tgctl_main(const std::vector<std::string>& args) { //NOLINT(readability-func
             return tateyama::session::session_shutdown(args.at(3));
         }
         if (args.at(2) == "set") {
-            if (args.size() < 6) {
-                std::cerr << "need to specify session-ref, set-key, and set-value" << std::endl;
+            if (args.size() < 5) {
+                std::cerr << "need to specify session-ref and set-key" << std::endl;
                 return tateyama::tgctl::return_code::err;
             }
+            if (args.size() < 6) {
+                return tateyama::session::session_swtch(args.at(3), args.at(4));
+            }
             return tateyama::session::session_swtch(args.at(3), args.at(4), args.at(5));
         }
         std::cerr << "unknown session sub command '" << args.at(2) << "'" << std::endl;
diff --git a/test/tateyama/session/session_test.cpp b/test/tateyama/session/session_test.cpp
index 683d629..0958db3 100644
--- a/test/tateyama/session/session_test.cpp
+++ b/test/tateyama/session/session_test.cpp
@@ -304,4 +304,31 @@ TEST_F(session_test, session_set) {
     EXPECT_EQ("test_value", rq.value());
 }
 
+TEST_F(session_test, session_unset) {
+    std::string command;
+    FILE *fp;
+
+    {
+        tateyama::proto::session::response::SessionSetVariable session_set{};
+        auto* success = session_set.mutable_success();
+        server_mock_->push_response(session_set.SerializeAsString());
+    }
+
+    command = "tgctl session set :123456 test_variable --conf ";
+    command += helper_->conf_file_path();
+    std::cout << command << std::endl;
+    if((fp = popen(command.c_str(), "r")) == nullptr){
+        std::cerr << "cannot tgctl session set" << std::endl;
+    }
+    auto result = read_pipe(fp);
+    std::cout << result << std::flush;
+
+    EXPECT_EQ(tateyama::framework::service_id_session, server_mock_->component_id());
+    tateyama::proto::session::request::SessionSetVariable rq{};
+    server_mock_->request_message(rq);
+    EXPECT_EQ(":123456", rq.session_specifier());
+    EXPECT_EQ("test_variable", rq.name());
+    EXPECT_EQ(tateyama::proto::session::request::SessionSetVariable::ValueOptCase::VALUE_OPT_NOT_SET, rq.value_opt_case());
+}
+
 }  // namespace tateyama::session