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ver : eccube-2_13
data/class_extends/page_extends/products/LC_Page_Products_List_Ex.php
に以下を追加

public function lfGetSearchConditionDisp($arrSearchData)
    {
        $objQuery   =& SC_Query_Ex::getSingletonInstance();
        $arrSearch  = array('category' => '指定なし', 'maker' => '指定なし', 'name' => '指定なし');
        // カテゴリ検索条件
        if ($arrSearchData['category_id'] > 0) {
            // category_id IN (1,2,3)
            $arrSearch['category']  = $objQuery->get('category_name', 'dtb_category', 'category_id IN (?)', array(implode(',', $arrSearchData['category_id'])));
        }

        return $arrSearch;
    }