#nav-open {
    display: inline-block;
    width: 30px;
    vertical-align: middle;
    padding-left: 5px;
    margin-top: 8px;
}

#nav-open span, #nav-open span:before, #nav-open span:after {
    position: absolute;
    height: 3px;/*線の太さ*/
    width: 25px;/*長さ*/
    border-radius: 3px;
    background: #555;
    display: block;
    content: '';
    cursor: pointer;
}
#nav-open span:before {
    bottom: -8px;
}
#nav-open span:after {
    bottom: -16px;
}