->addEventListener(FormEvents::POST_SET_DATA, function ($event) use($app) {
                $form = $event->getForm();

                $Categories = $this->app['eccube.repository.category']->findAll();

                $category_ids = new \Doctrine\Common\Collections\ArrayCollection();
                foreach ($Categories as $Category) {
                    $Path = $Category->getPath();
                    foreach ($Path as $Cat) {
                        if ($Cat->getId() == 4) {   // 特定カテゴリー配下のカテゴリーのみ
                            $category_ids->add($Category);
                            break;
                        }
                    }
                }

                $form['category_id']->setData($category_ids);
            })

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $app = $this->app;

        /**
         * @var ArrayCollection $arrCategory array of category
         */
        $Parent = $this->app['eccube.repository.category']->find(4);    // 特定カテゴリー配下のカテゴリーのみ
        $arrCategory = $this->app['eccube.repository.category']->getList($Parent, true);

        $builder

            ---(中略)---

            ->add('category_id', 'category', array(
                'label' => 'カテゴリ',
                'empty_value' => '選択してください',
                'required' => false,
                'choices' => $arrCategory,    // カテゴリーを設定
            ))