＜CSS部分＞
<style>
.google-maps {
position: relative;
padding-bottom: 75%; // これが縦横比
height: 0;
overflow: hidden;
}
.google-maps iframe {
position: absolute;
top: 0;
left: 0;
width: 100% !important;
height: 100% !important;
}
</style>
＜HTML部分＞
<div class=”google-maps”>
<iframe src=”https://www.google.com/maps/embed?pb=!1m14!1m12!1m3!1d7098.94326104394!2d78.0430654485247!3d27.172909818538997!2m3!1f0!2f0!3f0!3m2!1i1024!2i768!4f13.1!5e0!3m2!1sen!2s!4v1385710909804″ width=”600″ height=”450″ frameborder=”0″ style=”border:0″></iframe>
</div>
参照：http://www.lifehacker.jp/2013/12/131209google_maps_responsive.html