//全ての親ページをobjectで取得
        function get_parent_posts($post_id) {
            $post = get_post($post_id);
            if(empty($post_id) or $post->post_parent == '0') {
                return false;
            }
            while ($post_id){
                $post = get_post($post->post_parent);
                $result[] = $post;
                $post_id = $post->post_parent;
            }
            $result = array_reverse( $result );
            return $result;
        }

    $page_parents = get_parent_posts($post->ID);
    if ($page_parents){
        echo '<ul>';
        foreach ($page_parents as $page) {
            echo '<li>'.$page->post_title.'</li>';
        }
        echo '</ul>';
    }