diff --git a/Cask b/Cask
index 3c10a5e..d5a854a 100644
--- a/Cask
+++ b/Cask
@@ -2,6 +2,7 @@
 (source "melpa" "https://melpa.org/packages/")
 
 (package-file "phpstan.el")
+(package-file "phpstan-flycheck.el")
 (development
  (depends-on "flycheck")
  (depends-on "php-mode"))
diff --git a/phpstan-flycheck.el b/phpstan-flycheck.el
new file mode 100644
index 0000000..9c1ea87
--- /dev/null
+++ b/phpstan-flycheck.el
@@ -0,0 +1,67 @@
+;;; phpstan-flycheck.el --- Flycheck integration for PHPStan  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2018  Friends of Emacs-PHP development
+
+;; Author: USAMI Kenta <tadsan@zonu.me>
+;; Created: 15 Mar 2018
+;; Version: 0.1.0
+;; Keywords: convenience, php
+;; Homepage: https://github.com/emacs-php/phpstan.el
+;; Package-Requires: ((emacs "24") (flycheck "26"))
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Flycheck integration for PHPStan.
+
+;;; Code:
+(require 'flycheck)
+(require 'phpstan)
+
+;; Usually it is defined dynamically by flycheck
+(defvar flycheck-phpstan-executable)
+
+(defun phpstan-flycheck--enabled-and-set-variable ()
+  "Return path to phpstan configure file, and set buffer execute in side effect."
+  (let ((enabled (not (null (or phpstan-working-dir (phpstan-get-config-file))))))
+    (prog1 enabled
+      (when (and phpstan-flycheck-auto-set-executable
+                 (not (and (boundp 'flycheck-phpstan-executable)
+                           (symbol-value 'flycheck-phpstan-executable)))
+                 (or (eq 'docker phpstan-executable)
+                     (and (consp phpstan-executable)
+                          (stringp (car phpstan-executable))
+                          (listp (cdr phpstan-executable)))))
+        (set (make-local-variable 'flycheck-phpstan-executable)
+             (if (eq 'docker phpstan-executable)
+                 phpstan-docker-executable
+               (car phpstan-executable)))))))
+
+;;;###autoload
+(flycheck-define-checker phpstan
+  "PHP static analyzer based on PHPStan."
+  :command ("php" (eval (phpstan-get-command-args))
+            (eval (phpstan-normalize-path
+                   (flycheck-save-buffer-to-temp #'flycheck-temp-file-inplace)
+                   (flycheck-save-buffer-to-temp #'flycheck-temp-file-system))))
+  :working-directory (lambda (_) (phpstan-get-working-dir))
+  :enabled (lambda () (phpstan-flycheck--enabled-and-set-variable))
+  :error-patterns
+  ((error line-start (1+ (not (any ":"))) ":" line ":" (message) line-end))
+  :modes (php-mode)
+  :next-checkers (php))
+
+(provide 'phpstan-flycheck)
+;;; phpstan-flycheck.el ends here
diff --git a/phpstan.el b/phpstan.el
index de7986e..a16110e 100644
--- a/phpstan.el
+++ b/phpstan.el
@@ -29,7 +29,6 @@
 
 ;;; Code:
 (require 'php-project)
-(require 'flycheck nil)
 
 
 ;; Variables:
@@ -116,9 +115,6 @@ max
 
 (defconst phpstan-docker-executable "docker")
 
-;; Usually it is defined dynamically by flycheck
-(defvar flycheck-phpstan-executable)
-
 ;;;###autoload
 (progn
   (defvar phpstan-executable nil
@@ -166,22 +162,6 @@ NIL
                if dir
                return (expand-file-name name dir)))))
 
-(defun phpstan-enabled-and-set-flycheck-variable ()
-  "Return path to phpstan configure file, and set buffer execute in side effect."
-  (let ((enabled (not (null (or phpstan-working-dir (phpstan-get-config-file))))))
-    (prog1 enabled
-      (when (and phpstan-flycheck-auto-set-executable
-                 (not (and (boundp 'flycheck-phpstan-executable)
-                           (symbol-value 'flycheck-phpstan-executable)))
-                 (or (eq 'docker phpstan-executable)
-                     (and (consp phpstan-executable)
-                          (stringp (car phpstan-executable))
-                          (listp (cdr phpstan-executable)))))
-        (set (make-local-variable 'flycheck-phpstan-executable)
-             (if (eq 'docker phpstan-executable)
-                 phpstan-docker-executable
-               (car phpstan-executable)))))))
-
 (defun phpstan-normalize-path (source-original &optional source)
   "Return normalized source file path to pass by `SOURCE-ORIGINAL' OR `SOURCE'.
 
@@ -245,20 +225,5 @@ it returns the value of `SOURCE' as it is."
       (setq args (append args (list "-l" level))))
     (append executable args)))
 
-;;;###autoload
-(when (featurep 'flycheck)
-  (flycheck-define-checker phpstan
-    "PHP static analyzer based on PHPStan."
-    :command ("php" (eval (phpstan-get-command-args))
-              (eval (phpstan-normalize-path
-                     (flycheck-save-buffer-to-temp #'flycheck-temp-file-inplace)
-                     (flycheck-save-buffer-to-temp #'flycheck-temp-file-system))))
-    :working-directory (lambda (_) (phpstan-get-working-dir))
-    :enabled (lambda () (phpstan-enabled-and-set-flycheck-variable))
-    :error-patterns
-    ((error line-start (1+ (not (any ":"))) ":" line ":" (message) line-end))
-    :modes (php-mode)
-    :next-checkers (php)))
-
 (provide 'phpstan)
 ;;; phpstan.el ends here