diff --git a/app/components/my-notebooks/ImportCardsForm.tsx b/app/components/my-notebooks/ImportCardsForm.tsx
index 7bb1e95..2bd97e8 100644
--- a/app/components/my-notebooks/ImportCardsForm.tsx
+++ b/app/components/my-notebooks/ImportCardsForm.tsx
@@ -1,6 +1,7 @@
 import { useState } from "react";
 import { useFetcher } from "react-router";
 
+import { MAX_EXCEL_UPLOAD_SIZE } from "@/lib/shared/excel-constraints";
 import type { FormState } from "@/lib/shared/form-state";
 
 const initialState: FormState = {};
@@ -174,6 +175,9 @@ export default function ImportCardsForm({ notebookId }: { notebookId: string })
           required
           className="text-sm text-zinc-600 file:mr-3 file:rounded-full file:border-0 file:bg-coral-100 file:px-3 file:py-1.5 file:text-sm file:font-medium file:text-coral-700 hover:file:bg-coral-200 hover:file:text-coral-800 dark:text-zinc-400 dark:file:bg-coral-900/30 dark:file:text-coral-300 dark:hover:file:bg-coral-900/50 dark:hover:file:text-coral-200"
         />
+        <p className="text-xs text-zinc-500 dark:text-zinc-500">
+          ファイルサイズは最大 {MAX_EXCEL_UPLOAD_SIZE / (1024 * 1024)} MB です。
+        </p>
         <p className="text-xs text-zinc-500 dark:text-zinc-500">
           1行目を見出し行として読み取ります。既存の列と同じ名前の列はそこに追加され、無い列名は新しい列として追加されます。
         </p>
diff --git a/app/lib/server/my-notebooks/actions.server.test.ts b/app/lib/server/my-notebooks/actions.server.test.ts
index d9f6f74..dab9b52 100644
--- a/app/lib/server/my-notebooks/actions.server.test.ts
+++ b/app/lib/server/my-notebooks/actions.server.test.ts
@@ -20,11 +20,13 @@ import {
   MAX_STAR_COUNT,
   NOTEBOOK_TITLE_MAX_LENGTH,
 } from "@/lib/shared/notebook-schema";
+import { EXCEL_UPLOAD_SIZE_ERROR, MAX_EXCEL_UPLOAD_SIZE } from "@/lib/shared/excel-constraints";
 
 import {
   addNotebookColumn,
   createBlankNotebook,
   createCard as createCardAction,
+  importCardsFromExcel,
   deleteCard,
   deleteNotebook,
   moveCard,
@@ -585,6 +587,17 @@ describe("★(toggleStar / setStarCount)", () => {
 });
 
 describe("入力の検証", () => {
+  it("10 MB を超える Excel ファイルは解析せず拒否する", async () => {
+    loginAs(owner);
+    const formData = new FormData();
+    formData.set("file", new File([new Uint8Array(MAX_EXCEL_UPLOAD_SIZE + 1)], "large.xlsx"));
+
+    expect(await importCardsFromExcel(notebook.id, formData)).toEqual({
+      error: EXCEL_UPLOAD_SIZE_ERROR,
+    });
+    expect(await prisma.card.count({ where: { notebookId: notebook.id } })).toBe(0);
+  });
+
   it("長すぎるタイトルの単語帳は作成しない", async () => {
     loginAs(owner);
     const before = await prisma.notebook.count({ where: { userId: owner.id } });
diff --git a/app/lib/server/my-notebooks/actions.server.ts b/app/lib/server/my-notebooks/actions.server.ts
index 6b7634e..61775a1 100644
--- a/app/lib/server/my-notebooks/actions.server.ts
+++ b/app/lib/server/my-notebooks/actions.server.ts
@@ -4,6 +4,7 @@ import { prisma } from "@/lib/server/prisma.server";
 import { requireUser } from "@/lib/server/session.server";
 import { requireNotebookRole } from "@/lib/server/notebook-access.server";
 import { ExcelParseError, parseExcelWorkbook } from "@/lib/shared/excel";
+import { EXCEL_UPLOAD_SIZE_ERROR, MAX_EXCEL_UPLOAD_SIZE } from "@/lib/shared/excel-constraints";
 import { normalizeCardData, type CardData } from "@/lib/shared/card-data";
 import {
   DEFAULT_HEAD_COLUMN,
@@ -96,6 +97,9 @@ export async function importCardsFromExcel(
   if (!(file instanceof File) || file.size === 0) {
     return { error: "Excelファイル（.xlsx）を選択してください。" };
   }
+  if (file.size > MAX_EXCEL_UPLOAD_SIZE) {
+    return { error: EXCEL_UPLOAD_SIZE_ERROR };
+  }
 
   const notebook = await prisma.notebook.findUniqueOrThrow({
     where: { id: notebookId },
diff --git a/app/lib/shared/excel-constraints.ts b/app/lib/shared/excel-constraints.ts
new file mode 100644
index 0000000..f222f6a
--- /dev/null
+++ b/app/lib/shared/excel-constraints.ts
@@ -0,0 +1,2 @@
+export const MAX_EXCEL_UPLOAD_SIZE = 10 * 1024 * 1024;
+export const EXCEL_UPLOAD_SIZE_ERROR = "Excelファイルは10 MB以下にしてください。";
diff --git a/app/lib/shared/excel.test.ts b/app/lib/shared/excel.test.ts
index b766826..2973b00 100644
--- a/app/lib/shared/excel.test.ts
+++ b/app/lib/shared/excel.test.ts
@@ -2,6 +2,7 @@ import ExcelJS from "exceljs";
 import { describe, expect, it } from "vite-plus/test";
 
 import { createTemplateWorkbookBuffer, ExcelParseError, parseExcelWorkbook } from "./excel";
+import { MAX_EXCEL_UPLOAD_SIZE } from "./excel-constraints";
 
 async function workbookBuffer(rows: (string | number | null)[][]): Promise<ArrayBuffer> {
   const workbook = new ExcelJS.Workbook();
@@ -55,6 +56,12 @@ describe("parseExcelWorkbook", () => {
     );
   });
 
+  it("10 MB を超えるファイルは解析前に拒否する", async () => {
+    await expect(parseExcelWorkbook(new ArrayBuffer(MAX_EXCEL_UPLOAD_SIZE + 1))).rejects.toThrow(
+      ExcelParseError,
+    );
+  });
+
   it("xlsx でないデータのエラーでは、Googleスプレッドシートを .xlsx で書き出す方法を案内する", async () => {
     // Googleスプレッドシートから CSV などで書き出したファイルを選んだときを想定する
     await expect(
diff --git a/app/lib/shared/excel.ts b/app/lib/shared/excel.ts
index 51faff6..7241ac9 100644
--- a/app/lib/shared/excel.ts
+++ b/app/lib/shared/excel.ts
@@ -1,6 +1,9 @@
 import ExcelJS from "exceljs";
 
 import type { CardData } from "@/lib/shared/card-data";
+import { EXCEL_UPLOAD_SIZE_ERROR, MAX_EXCEL_UPLOAD_SIZE } from "@/lib/shared/excel-constraints";
+
+export { EXCEL_UPLOAD_SIZE_ERROR, MAX_EXCEL_UPLOAD_SIZE } from "@/lib/shared/excel-constraints";
 
 export type ParsedNotebook = {
   // Excelの1行目（ヘッダー行）の値を、出現順のまま並べたもの。
@@ -52,6 +55,10 @@ function cellToText(value: ExcelJS.CellValue): string {
 
 // アップロードされたExcelファイルの1シート目を読み取り、1行目をヘッダーとする可変列の単語帳データに変換する
 export async function parseExcelWorkbook(buffer: ArrayBuffer): Promise<ParsedNotebook> {
+  if (buffer.byteLength > MAX_EXCEL_UPLOAD_SIZE) {
+    throw new ExcelParseError(EXCEL_UPLOAD_SIZE_ERROR);
+  }
+
   // ファイルを読み込み、データをworkbookに格納する
   const workbook = new ExcelJS.Workbook();
   try {