Index: branches/version-2_12-dev/data/Smarty/templates/default/products/list.tpl
===================================================================
--- branches/version-2_12-dev/data/Smarty/templates/default/products/list.tpl	(revision 22206)
+++ branches/version-2_12-dev/data/Smarty/templates/default/products/list.tpl	(revision 22607)
@@ -197,4 +197,5 @@
                 <input type="hidden" name="product_id" value="<!--{$id|h}-->" />
                 <input type="hidden" name="product_class_id" id="product_class_id<!--{$id|h}-->" value="<!--{$tpl_product_class_id[$id]}-->" />
+                <input type="hidden" name="category_id" value="<!--{$arrSearchData.category_id|h}-->" />
 
                 <div class="cart_area clearfix">
Index: branches/version-2_12-dev/data/class/pages/products/LC_Page_Products_List.php
===================================================================
--- branches/version-2_12-dev/data/class/pages/products/LC_Page_Products_List.php	(revision 22595)
+++ branches/version-2_12-dev/data/class/pages/products/LC_Page_Products_List.php	(revision 22607)
@@ -536,6 +536,10 @@
                 $this->lfAddCart($this->arrForm, $_SERVER['HTTP_REFERER']);
 
-
-                SC_Response_Ex::sendRedirect(CART_URLPATH);
+                // 開いているカテゴリーツリーを維持するためのパラメーター
+                $arrQueryString = array(
+                    'category_id' => $this->arrForm['category_id'],
+                );
+
+                SC_Response_Ex::sendRedirect(CART_URLPATH, $arrQueryString);
                 SC_Response_Ex::actionExit();
             }