diff --git a/.husky/pre-commit b/.husky/pre-commit
index 2cbae3d..c345039 100755
--- a/.husky/pre-commit
+++ b/.husky/pre-commit
@@ -7,3 +7,4 @@ npm test
 npx lint-staged
 
 npm run checkLicense
+npm run checkSecrets
diff --git a/README.md b/README.md
index ce1db76..ee9c6a5 100644
--- a/README.md
+++ b/README.md
@@ -16,6 +16,10 @@ Copyright (C) 2023 Shusui Moyatani
 
 AGPL-3.0-or-later
 
+Please note that some files are licensed in other FLOSS licenses.
+Those files contain the license text in the first lines.
+If the file does not have such text, it must be considered licensed in AGPL.
+
 ### English
 
 This program is free software: you can redistribute it and/or modify
diff --git a/package.json b/package.json
index f417687..5f424e9 100644
--- a/package.json
+++ b/package.json
@@ -17,7 +17,8 @@
     "cover": "vitest run --coverage",
     "prepare": "husky install",
     "generatePackageInfo": "node -e 'import(\"./scripts/generatePackageInfo.mjs\").then((m) => m.default())'",
-    "checkLicense": "node -e 'import(\"./scripts/checkLicense.mjs\").then((m) => m.default())'"
+    "checkLicense": "node -e 'import(\"./scripts/checkLicense.mjs\").then((m) => m.default())'",
+    "checkSecrets": "node -e 'import(\"./scripts/checkSecrets.mjs\").then((m) => m.default())'"
   },
   "devDependencies": {
     "@typescript-eslint/eslint-plugin": "^5.59.7",
diff --git a/scripts/checkSecrets.mjs b/scripts/checkSecrets.mjs
new file mode 100644
index 0000000..b0838b7
--- /dev/null
+++ b/scripts/checkSecrets.mjs
@@ -0,0 +1,82 @@
+/**
+ * To the extent possible under law, the person who associated CC0
+ * with this file has waived all copyright and related or
+ * neighboring rights to this work.
+ *
+ * https://creativecommons.org/publicdomain/zero/1.0/
+ */
+import fs from 'fs/promises';
+import path from 'path';
+import util from 'util';
+
+const rootDir = path.resolve();
+const pattern = /nsec1[0-9a-zA-Z]+/;
+const ignored = [/^node_modules$/, /^\./, /\.tsbuildinfo$/, /^public$/, /^dist$/];
+
+const ignoreNextLine = /@check-secrets-disable-next-line/;
+
+const shouldIgnore = (filename) => ignored.some((pattern) => pattern.test(filename));
+
+const searchFiles = async (folderPath) => {
+  let didMatch = false;
+
+  const files = await fs.readdir(folderPath);
+
+  for (const file of files) {
+    const filePath = path.join(folderPath, file);
+    const stats = await fs.stat(filePath);
+
+    if (shouldIgnore(file)) continue;
+
+    if (stats.isDirectory()) {
+      const match = await searchFiles(filePath);
+      didMatch ||= match;
+    } else {
+      const match = await checkKeyword(filePath);
+      didMatch ||= match;
+    }
+  }
+
+  return didMatch;
+};
+
+const checkKeyword = async (filePath) => {
+  const content = await fs.readFile(filePath, 'utf8');
+  const lines = content.split('\n');
+  let prevLine = '';
+  let didMatch = false;
+  let didShowFilename = false;
+
+  for (let i = 0; i < lines.length; i++) {
+    const line = lines[i];
+
+    const match = line.match(pattern);
+    if (match != null) {
+      if (!didShowFilename) {
+        console.error(filePath);
+        didShowFilename = true;
+      }
+      if (ignoreNextLine.test(prevLine.trim())) {
+        console.error(`ignored: ${i + 1}: "${match[0]}": ${line}`);
+        continue;
+      }
+      console.error(`${i + 1}: "${match[0]}": ${line}`);
+      didMatch = true;
+    }
+
+    prevLine = line;
+  }
+
+  return didMatch;
+};
+
+const main = async () => {
+  const result = await searchFiles(rootDir);
+  if (result) {
+    process.exit(1);
+  } else {
+    process.exit(0);
+  }
+};
+
+export default main;