diff --git a/src/index.ts b/src/index.ts
index 7c4e165..f54d4f3 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -12,7 +12,8 @@ function doPost(e: GoogleAppsScript.Events.DoPost) {
   const req: AlertRequest = JSON.parse(e.postData.contents)
   const alertMessages = req.alerts.map(alertToMessage)
   const text = alertMessages.join('\n\n')
-  sendMessage(text)
+  const channelID = e.parameter.channelID as string | undefined
+  sendMessage(text, channelID)
 }
 
 function getStatusEmoji(status: string) {
@@ -37,24 +38,32 @@ ${link}
 `.trim()
 }
 
-function sendMessage(message: string) {
+function sendMessage(message: string, channelID: string | undefined) {
+  const sign = computeSignature(message)
+  const headers: Record<string, string> = {
+    'Content-Type': 'text/plain; charset=utf-8',
+    'X-TRAQ-Signature': sign
+  }
+  if (channelID !== undefined) {
+    headers['X-TRAQ-Channel-Id'] = channelID
+  }
+  UrlFetchApp.fetch(`https://q.trap.jp/api/v3/webhooks/${WEBHOOK_ID}`, {
+    method: 'post',
+    contentType: 'text/plain; charset=utf-8',
+    headers: headers,
+    payload: message
+  })
+}
+
+function computeSignature(message: string) {
   const signature = Utilities.computeHmacSignature(
     Utilities.MacAlgorithm.HMAC_SHA_1,
     message,
     WEBHOOK_SECRET,
     Utilities.Charset.UTF_8
   )
-  const sign = signature.reduce((str, ch) => {
+  return signature.reduce((str, ch) => {
     const chr = (ch < 0 ? ch + 256 : ch).toString(16)
     return str + (chr.length === 1 ? '0' : '') + chr
   }, '')
-  UrlFetchApp.fetch(`https://q.trap.jp/api/v3/webhooks/${WEBHOOK_ID}`, {
-    method: 'post',
-    contentType: 'text/plain; charset=utf-8',
-    headers: {
-      'Content-Type': 'text/plain; charset=utf-8',
-      'X-TRAQ-Signature': sign
-    },
-    payload: message
-  })
 }