メガメニュー（レスポンシブ）

サンプルはこちら

PCではメガメニュー、SPの時はアコーディオンに同一メニューで設定しています。

HTMLはこちら

<header class="l-header">
        <div class="l-header__inner">
          <h1 class="header-logo"><img src="images/logo.jpg" alt="logo"></h1>
          <button type="button" class="sp-menubtn" aria-label="メニューを開く"><span></span><span></span><span></span></button>
        </div>
        <nav class="menu" aria-label="サイトメニュー">
          <ul class="menu__inner" id="menu">
            <li class="menu__item">
              <a href="#" class="menu__link">MENU1</a>
              <div class="menu__contents">
                <ul>
                  <li><a href="#">MENU1-1</a></li>
                  <li><a href="#">MENU1-2</a></li>
                  <li><a href="#">MENU1-3</a></li>
                </ul>
              </div>
            </li>
            <li class="menu__item">
              <a href="#" class="menu__link">MENU2</a>
              <div class="menu__contents">
                <ul>
                  <li><a href="#">MENU2-1</a></li>
                  <li><a href="#">MENU2-2</a></li>
                  <li><a href="#">MENU2-3</a></li>
                </ul>
              </div>
            </li>
            <li class="menu__item">
              <a href="#" class="menu__link">MENU3</a>
              <div class="menu__contents">
                <ul>
                  <li><a href="#">MENU3-1</a></li>
                  <li><a href="#">MENU3-2</a></li>
                  <li><a href="#">MENU3-3</a></li>
                </ul>
              </div>
            </li>
            <li class="menu__item">
              <a href="#" class="menu__link">MENU4</a>
              <div class="menu__contents">
                <ul>
                  <li><a href="#">MENU4-1</a></li>
                  <li><a href="#">MENU4-2</a></li>
                  <li><a href="#">MENU4-3</a></li>
                </ul>
              </div>
            </li>
            <li class="menu__item">
              <a href="#" class="menu__link">MENU5</a>
              <div class="menu__contents">
                <ul>
                  <li><a href="#">MENU5-1</a></li>
                  <li><a href="#">MENU5-2</a></li>
                  <li><a href="#">MENU5-3</a></li>
                </ul>
              </div></li>
            <li class="menu__contact"><a href="#" class="menu__link"><span>CONTACT<span class="sp-only-2">&nbsp;US</span></span></a></li>          
          </ul>
        </nav>
      </header>

PC用のcssはこちら。

.l-header{
	width:100%;
	height:85px;
	display: flex;
	justify-content: space-between;
	align-items: center;
}
.l-header__inner:before {
	content: "";
	width: 100%;
	height: 85px;
	background: linear-gradient(43.37deg, #4d4d4d 6.71%, #bcbbbb 101.43%);
	position: absolute;
	top: 0;
	left: 0;
	z-index: -100;
}
.sp-menubtn{
	display: none;
}
.menu__inner{
	display: flex;
	justify-content: flex-end;
	align-items: center;
	height:85px;	
	position:relative;
}
.menu__item{
	margin:0 20px;
	height:100%;
}
.menu__contact{
	background: #4d4d4d;
	width: 150px;
	text-align: center;
}
.menu__link{
	display:block;
	color:#fff;
	height:100%;
	line-height: 85px;
}
/*メガメニュー*/
.menu__contents{
	display: none;
	position:absolute;
	top:100%;
	left:0;
	width: calc(100% - 150px);
	background: #bcbbbb;
	padding:20px;	
}
.menu__contents ul {
	display: flex;
	justify-content: space-between;
}
.menu__contents ul li{
	background:#fff;
	width: 32%;
	height:100px;
}
.menu__contents ul li a{
	display:block;
	text-align: center;
	line-height: 100px;
}

そしてSP用のcss

@media screen and (max-width: 768px) {
	.header-logo img{
		height:60px;
	}
	.l-header{
		height:60px;
	}
	.l-header__inner:before {
		height: 60px;
	}
	.l-header__inner{
		width:100%;
		height:60px;
		display: flex;
		justify-content: space-between;
		align-items: center;
	}
	.sp-menubtn {
		width: 60px;
		height: 100%;
		position: relative;
		display: block;
		z-index: 5000;
		border: none;
		background: linear-gradient(64.63deg, #4d4d4d 6.71%, #bcbbbb 101.43%);
		cursor: pointer;
	}	
	.sp-menubtn span {
		display: block;
		overflow: hidden;
		white-space: nowrap;
		text-indent: 100%;
		width: 22px;
		height: 3px;
		background: #FFFFFF;
		position: absolute;
		transition: 0.2s;
		left: 50%;
		top: 50%;
	}	
	.sp-menubtn span:nth-child(1) {
		transform: translate(-50%, -9px);
	}	
	.sp-menubtn span:nth-child(2) {
		transform: translate(-50%, -50%);
	}	
	.sp-menubtn span:nth-child(3) {
		transform: translate(-50%, 6px);
	}	
	.sp-menubtn.active span {
		width: 25px;
		height: 2px;
	}	
	.sp-menubtn.active span:nth-child(1) {
		transform: translate(-50%, 50%) rotate(45deg);
	}	
	.sp-menubtn.active span:nth-child(2) {
		opacity: 0;
		left: 50%;
	}	
	.sp-menubtn.active span:nth-child(3) {
		transform: translate(-50%, 50%) rotate(-45deg);
	}
	.menu {
		position: fixed;
		top: 60px;
		left: 0;
		width: 100%;
		height: calc(100% - 60px);
		background: #4d4d4d;
		color: #FFFFFF;
		overflow: auto;
		opacity: 0;
		visibility: hidden;
		transition: opacity .6s ease, visibility .6s ease;
	}
	.menu.active{
		opacity:100;
		visibility: visible;
	}
	.menu__inner {
		display:block;
		height:auto;
		width: 72.16vw;
		margin-left: auto;
		margin-right: auto;
		padding: 40px 0 60px;
	}	
	.menu__item {
		border-bottom:#bcbbbb solid 2px;
		width: 100%;
		position: relative;
	}		
	.menu__link {
		font-weight: bold;
		font-size: 1.6rem;
		letter-spacing: 0.04em;
		height: 80px;
		align-items: center;
	}	
	.menu__contact {
		width: 72.16vw;
		margin-top: 36px;
	}	
	.menu__contact a {
		background: linear-gradient(58.5deg, #4d4d4d 46.25%, #bcbbbb 109.07%);
		color: #FFFFFF;
		display: -webkit-box;
		display: -ms-flexbox;
		display: flex;
		-webkit-box-align: center;
		-ms-flex-align: center;
		align-items: center;
		height: 68px;
		padding: 0 35px 0 30px;
		position: relative;
		letter-spacing: 0.04em;
		max-width: 72.16vw;
		margin-left: auto;
		margin-right: auto;
		box-shadow: 0px 0px 22px rgba(0, 0, 0, 0.1);
	}	
	.menu__contact a span {
		position: relative;
	}	
	.menu__contact a:before {
		position: absolute;
		top: 0;
		right: 0;
		bottom: 0;
		left: 0;
		content: '';
		background: #01397E;;
		transform-origin: right top;
		transform: scale(0, 1);
		transition: -webkit-transform .3s;
		transition: transform .3s;
	}	
	.menu__contact a:after {
		content: "";
		display: block;
		width: 7px;
		height: 7px;
		border-top: 2px solid #FFFFFF;
		border-right: 2px solid #FFFFFF;
		transform: rotate(45deg);
		position: absolute;
		top: 50%;
		right: 30px;
		transform: translate(-50%, -50%) rotate(45deg);
	}	
		/*メガメニュー　→アコーディオン*/
	.menu__contents{
		display: none;
		position:static;
		width:100%;
		background: #cccccc;
		padding:20px;	
	}
	.menu__contents ul {
		display: block;
	}
	.menu__contents ul li{
		border-bottom:#000;
		width: 100%;
		height:auto;
	}
	.menu__contents ul li a{
		color:#000;
		display:block;
		text-align: center;
		line-height:2.0;
	}
	.menu__item::after{
		content: "";
		position: absolute;
		right: 25px;
		top:35px;
		transform:translateY(-50%);
		transition: all 0.2s ease-in-out;
		display: block;
		width: 10px;
		height: 10px;
		border-top: solid 2px #E5E5E5;
		border-right: solid 2px #E5E5E5;
		-webkit-transform: rotate(135deg);
		transform: rotate(135deg);
	}
	.menu__item.active::after{
		content: "";
		position: absolute;
		right: 25px;
		top:35px;
		transform:translateY(-50%);
		transition: all 0.2s ease-in-out;
		display: block;
		width: 10px;
		height:10px;
		border-top: solid 2px #E5E5E5;
		border-right: solid 2px #E5E5E5;
		-webkit-transform: rotate(-45deg);
		transform: rotate(-45deg);
	}
}

最後にjs。（jQueryを別途読み込んで使用してください。）

//ハンバーガーメニュー
$(function() {	
	$('.sp-menubtn').click(function() {
			$(this).toggleClass('active');

			if ($(this).hasClass('active')) {
					$('.menu').addClass('active');
			} else {
					$('.menu').removeClass('active');
			}
	});	
});
//SP時アコーディオン＆PC時メガメニュー
$(window).on('load resize', function(){
		var winW = $(window).width();
		var devW = 767;
		if (winW <= devW) {
			$(function(){
					$(".menu__item").on("click", function() {
							$(this).children('.menu__contents').slideToggle();
							$(this).toggleClass("active");
					});
			});
		} else {
			$(function () {
				$('.menu__item').hover(function(){
						$(this).children('.menu__contents').stop().slideToggle();
				});
			});
		}
	});

jsでウィンドウサイズを読み込み、767px以上（PC時）ではホバーしたときに反応、
767px以下（SP時）ではクリックしたときに反応するように設定しています。

サンプルはこちら。