diff --git a/src/domain/post.test.ts b/src/domain/post.test.ts
new file mode 100644
index 0000000..9431915
--- /dev/null
+++ b/src/domain/post.test.ts
@@ -0,0 +1,38 @@
+import { describe, expect, it } from "vitest";
+import { Post, PostArgs } from "./post.js";
+import { Snowflake } from "../helpers/id_generator.js";
+import { Media } from "./media.js";
+
+describe("Post", () => {
+  const defaultPostArgs: PostArgs = {
+    id: "10101450391945216" as Snowflake,
+    authorID: "10101451896913920" as Snowflake,
+    text: "Hello, world!",
+    visibility: 0,
+    createdAt: new Date(),
+    attachments: [],
+    reactions: [],
+  };
+
+  it("添付ファイルは16個以上添付できない", () => {
+    expect(() => {
+      new Post({
+        ...defaultPostArgs,
+        attachments: new Array(17).fill({} as Media),
+      });
+    }).toThrowError(
+      new Error(
+        "failed to create post: The number of attachments must be less than 16.",
+      ),
+    );
+  });
+
+  it("ファイルを添付できる", () => {
+    expect(() => {
+      new Post({
+        ...defaultPostArgs,
+        attachments: new Array(10).fill({} as Media),
+      });
+    }).toBeTruthy();
+  });
+});
diff --git a/src/domain/post.ts b/src/domain/post.ts
index 46bf32f..c949194 100644
--- a/src/domain/post.ts
+++ b/src/domain/post.ts
@@ -2,6 +2,16 @@ import { Snowflake } from "../helpers/id_generator.js";
 import { User } from "./user.js";
 import { Media } from "./media.js";
 
+export interface PostArgs {
+  id: Snowflake;
+  authorID: Snowflake;
+  text: string;
+  visibility: number;
+  createdAt: Date;
+  attachments: Array<Media>;
+  reactions: Array<PostReactionEvent>;
+}
+
 export class Post {
   get visibility(): number {
     return this._visibility;
@@ -54,6 +64,7 @@ export class Post {
     attachments: Array<Media>;
     reactions: Array<PostReactionEvent>;
   }) {
+    this.validate(args);
     this._id = args.id;
     this._authorID = args.authorID;
 
@@ -70,6 +81,14 @@ export class Post {
     }
     return text;
   }
+
+  private validate(args: PostArgs) {
+    if (args.attachments.length > 16) {
+      throw new Error(
+        "failed to create post: The number of attachments must be less than 16.",
+      );
+    }
+  }
 }
 
 export class PostReactionEvent {