Wordpress导航菜单在ul之前添加div

I have some problems with nav menu in Wordpress. I have a nav-menu

<nav id="nav-menu">
<ul>
</ul>
</nav>

now I want add a div before and after ul:

<nav id="nav-menu">
<div></div>
<ul>
</ul>
<div></div>
</nav>

I've tried several hours but can't found the solution. Can anyone help me with this, thank you. I don't want to just display <ul></ul> by wp_nav_menu() and put HTML code <nav> and <div> before wp_nav_menu().

simply add this to your theme's functions.php file:

add_filter( 'wp_nav_menu_items', 'prefix_add_div', 10, 2 );

function prefix_add_div( $items, $args ) {
    $items = '<div></div>' . $items . '<div></div>';
    return $items;
}