diff --git a/src/endpoints/messages.tsp b/src/endpoints/messages.tsp
new file mode 100644
index 0000000..ee22b98
--- /dev/null
+++ b/src/endpoints/messages.tsp
@@ -0,0 +1,63 @@
+import "@typespec/http";
+import "@typespec/rest";
+import "../models/errors.tsp";
+import "../models/user.tsp";
+import "../models/message.tsp";
+
+using TypeSpec.Http;
+using TypeSpec.Rest;
+
+namespace SeichiPortalApiSchema;
+
+@tag("Messages")
+@route("/messages")
+namespace Messages {
+  @post
+  @summary("メッセージの新規作成")
+  op create(
+    @header
+    contentType: "application/json",
+
+    @body body: {
+      related_answer_id: uint32;
+      body: string;
+    },
+  ): {
+    @statusCode statusCode: 201;
+    @body body: {
+      id: uint32;
+    };
+  } | {
+    @statusCode statusCode: 400 | 401 | 403 | 500;
+    @body body: Error;
+  };
+
+  @get
+  @route("/{related_answer_id}")
+  @summary("メッセージの一覧取得")
+  op list(
+    @path
+    related_answer_id: uint32,
+  ): {
+    @statusCode statusCode: 200;
+    @body body: {
+      messages: Message[];
+    };
+  } | {
+    @statusCode statusCode: 400 | 401 | 403 | 500;
+    @body body: Error;
+  };
+
+  @delete
+  @route("/{message_id}")
+  @summary("メッセージの削除")
+  op delete(
+    @path
+    message_id: uint32,
+  ): {
+    @statusCode statusCode: 204;
+  } | {
+    @statusCode statusCode: 400 | 401 | 403 | 500;
+    @body body: Error;
+  };
+}
diff --git a/src/main.tsp b/src/main.tsp
index 84533fb..a1b15ce 100644
--- a/src/main.tsp
+++ b/src/main.tsp
@@ -1,4 +1,5 @@
 import "./endpoints/forms.tsp";
+import "./endpoints/messages.tsp";
 import "./endpoints/users.tsp";
 import "./endpoints/session.tsp";
 import "@typespec/openapi";
diff --git a/src/models/errors.tsp b/src/models/errors.tsp
index ce46954..137a936 100644
--- a/src/models/errors.tsp
+++ b/src/models/errors.tsp
@@ -1,6 +1,7 @@
 enum ErrorCode {
   FORM_NOT_FOUND,
   ANSWER_NOT_FOUND,
+  MESSAGE_NOT_FOUND,
   OUT_OF_PERIOD,
   DO_NOT_HAVE_PERMISSION_TO_POST_FORM_COMMENT,
   DO_NOT_HAVE_PERMISSION_TO_GET_ANSWERS,
diff --git a/src/models/message.tsp b/src/models/message.tsp
new file mode 100644
index 0000000..590badb
--- /dev/null
+++ b/src/models/message.tsp
@@ -0,0 +1,7 @@
+import "../endpoints/users.tsp";
+
+model Message {
+  body: string;
+  sender: User;
+  timestamp: utcDateTime;
+}