#key {
    background: url("../img/index/kv_img1_sp.jpg") no-repeat center/cover;
}

#sec1 p {
    text-align: center;
    margin: 50px auto;
}

@media only screen and (min-width: 768px) {
    #key {
        background: url("../img/index/kv_img1.jpg") no-repeat center/cover;
    }
}