diff --git a/.github/workflows/actions.yml b/.github/workflows/actions.yml
index 494a658d..1af51636 100644
--- a/.github/workflows/actions.yml
+++ b/.github/workflows/actions.yml
@@ -23,10 +23,16 @@ jobs:
     strategy:
       matrix:
         node-version:
-          - 14
-          - 16
           - 18
           - 20
+          - 21
+        eslint-version:
+          - 7
+          - 8
+          - 9
+
+    env:
+      ESLINT_VERSION: ${{ matrix.eslint-version }}
 
     steps:
       - uses: actions/checkout@v3
diff --git a/.npmrc b/.npmrc
index 43c97e71..74215ad6 100644
--- a/.npmrc
+++ b/.npmrc
@@ -1 +1,6 @@
 package-lock=false
+
+# As of 2024/04/10, typescript-eslint does not support ESLint v9 yet.
+# Therefore, you cannnot install `eslint` and `@typescript-eslint/*` packages at the same time for now.
+# Until typescript-eslint support ESLint v9, enable legacy-peer-deps.
+legacy-peer-deps=true
diff --git a/README.md b/README.md
index 9e6147bc..9844f01f 100644
--- a/README.md
+++ b/README.md
@@ -26,8 +26,13 @@ $ npm install --save-dev @typescript-eslint/eslint-plugin
 
 ### eslint.config.* (Flat Config)
 
-Unfortunately, eslint-plugin-editorconfig does not work properly with Flat Config for now, even if you use `@eslint/eslintrc` for compatibility.
-Please wait for the next version.
+Unfortunately, eslint-plugin-editorconfig does not work properly with Flat Config for now, even if you use `@eslint/eslintrc` for compatibility.  
+Please wait for the next version.  
+If you use ESLint v9, set the `ESLINT_USE_FLAT_CONFIG` environment variable to `false` to use traditional .eslintrc.\*.
+
+```shell
+ESLINT_USE_FLAT_CONFIG=false npx eslint --ext=.js,.ts .
+```
 
 ### .eslintrc.*
 
diff --git a/package.json b/package.json
index 4c2d052f..1b013ed8 100644
--- a/package.json
+++ b/package.json
@@ -5,7 +5,7 @@
   "main": "main.js",
   "scripts": {
     "test": "./test-packages/test.sh && mocha --reporter dot tests/lib/**/*.js",
-    "lint": "eslint .",
+    "lint": "ESLINT_USE_FLAT_CONFIG=false eslint .",
     "release": "npm publish . --access public"
   },
   "keywords": [
@@ -25,14 +25,13 @@
   "homepage": "https://github.com/phanect/eslint-plugin-editorconfig",
   "dependencies": {
     "editorconfig": "^1.0.2",
-    "eslint": "^8.40.0",
+    "eslint": "^9.0.0",
     "klona": "^2.0.4"
   },
   "devDependencies": {
-    "@typescript-eslint/eslint-plugin": "^5.0.0",
-    "@typescript-eslint/parser": "^5.0.0",
-    "eslint-config-phanective": "latest",
-    "mocha": "^10.2.0"
+    "@typescript-eslint/eslint-plugin": "^7.6.0",
+    "@typescript-eslint/parser": "^7.6.0",
+    "eslint-config-phanective": "latest"
   },
   "engines": {
     "node": ">=14",
diff --git a/test-packages/success/base-rules-conflict/package.json b/test-packages/success/base-rules-conflict/package.json
index c8fa2c5c..b6ba1b51 100644
--- a/test-packages/success/base-rules-conflict/package.json
+++ b/test-packages/success/base-rules-conflict/package.json
@@ -7,7 +7,7 @@
   "private": true,
   "main": "main.js",
   "scripts": {
-    "lint": "eslint . --ext=.js,.ts",
+    "lint": "ESLINT_USE_FLAT_CONFIG=false eslint . --ext=.js,.ts",
     "test": "npm run lint"
   }
 }
diff --git a/test-packages/success/js/package.json b/test-packages/success/js/package.json
index f99b1df2..3e97e443 100644
--- a/test-packages/success/js/package.json
+++ b/test-packages/success/js/package.json
@@ -7,7 +7,7 @@
   "private": true,
   "main": "main.js",
   "scripts": {
-    "lint": "eslint .",
+    "lint": "ESLINT_USE_FLAT_CONFIG=false eslint .",
     "test": "npm run lint"
   }
 }
diff --git a/test-packages/success/ts/package.json b/test-packages/success/ts/package.json
index 888b6a25..06840074 100644
--- a/test-packages/success/ts/package.json
+++ b/test-packages/success/ts/package.json
@@ -7,12 +7,12 @@
   "private": true,
   "main": "main.js",
   "scripts": {
-    "lint": "eslint . --ext=.js,.ts",
+    "lint": "ESLINT_USE_FLAT_CONFIG=false eslint . --ext=.js,.ts",
     "test": "npm run lint"
   },
   "devDependencies": {
-    "@typescript-eslint/eslint-plugin": "^5.0.0",
-    "@typescript-eslint/parser": "^5.0.0",
-    "typescript": "^4.1.3"
+    "@typescript-eslint/eslint-plugin": "^7.6.0",
+    "@typescript-eslint/parser": "^7.6.0",
+    "typescript": "^5.0.0"
   }
 }
diff --git a/test-packages/test.sh b/test-packages/test.sh
index 6166c038..59a160e6 100755
--- a/test-packages/test.sh
+++ b/test-packages/test.sh
@@ -2,10 +2,18 @@
 
 set -eux
 
+# Until this plugin supports flat config, test with legacy .eslintrc.*
+export ESLINT_USE_FLAT_CONFIG=false
+
 TEST_PKGS_DIR="$(realpath "$(dirname "${BASH_SOURCE[0]}")")"
 PROJECT_ROOT="$(realpath "${TEST_PKGS_DIR}/..")"
 TMP="$(mktemp --directory)"
 
+# To run tests on local machine
+if [[ -z "${ESLINT_VERSION:-}" ]]; then
+  ESLINT_VERSION=9
+fi
+
 cd "${PROJECT_ROOT}"
 PACKAGE="$(npm pack .)"
 
@@ -15,31 +23,25 @@ git clean -Xd --force
 # If you test without copying, test package is affected by node_modules in the project root
 cp --recursive "${TEST_PKGS_DIR}" "${TMP}/"
 
-for ESLINT_VERSION in "7" "8"; do
-  for PKGDIR in $(find "${TMP}/test-packages/success/" -maxdepth 1 -type d ! -path "${TMP}/test-packages/success/"); do
-    cd "${PKGDIR}"
-    npm install
-    npm install --save-dev "eslint@${ESLINT_VERSION}" "${PROJECT_ROOT}/${PACKAGE}"
+for PKGDIR in $(find "${TMP}/test-packages/success/" -maxdepth 1 -type d ! -path "${TMP}/test-packages/success/"); do
+  cd "${PKGDIR}"
+  npm install
+  npm install --save-dev "eslint@${ESLINT_VERSION}" "${PROJECT_ROOT}/${PACKAGE}"
+  npm run lint
+done
+
+for PKGDIR in $(find "${TMP}/test-packages/failure/" -maxdepth 1 -type d ! -path "${TMP}/test-packages/failure/"); do
+  cd "${PKGDIR}"
+  npm install
+  npm install --save-dev "eslint@${ESLINT_VERSION}" "${PROJECT_ROOT}/${PACKAGE}"
+
+  if \
+    [[ "${PKGDIR}" = "${TMP}/test-packages/failure/missing-ts-eslint" ]] && \
+    [[ -z "$((npm run lint 2>&1) | grep "eslint-plugin-editorconfig requires typescript and @typescript-eslint/eslint-plugin to lint \*.ts files. Run \`npm install typescript @typescript-eslint/eslint-plugin\`.")" ]]
+  then
+    echo "Error message is not shown properly when @typescript-eslint/eslint-plugin is missing" >&2
+    echo "ESLint's error message:"
     npm run lint
-  done
-
-  for PKGDIR in $(find "${TMP}/test-packages/failure/" -maxdepth 1 -type d ! -path "${TMP}/test-packages/failure/"); do
-    cd "${PKGDIR}"
-    npm install
-    npm install --save-dev "eslint@${ESLINT_VERSION}" "${PROJECT_ROOT}/${PACKAGE}"
-
-    if [[ "${PKGDIR}" = "${TMP}/test-packages/failure/missing-ts-eslint" ]]; then
-      # skip if Node.js version == 14.x
-      if [[ -n "$(node --version | grep '^v14\.')" ]]; then
-        continue
-      fi
-
-      if [[ -z "$((npm run lint 2>&1) | grep "eslint-plugin-editorconfig requires typescript and @typescript-eslint/eslint-plugin to lint \*.ts files. Run \`npm install typescript @typescript-eslint/eslint-plugin\`.")" ]]; then
-        echo "Error message is not shown properly when @typescript-eslint/eslint-plugin is missing" >&2
-        echo "ESLint's error message:"
-        npm run lint
-        exit 1
-      fi
-    fi
-  done
+    exit 1
+  fi
 done