<ul class="menu">
  <li><a href="#">メニュー１ ▼</a>
    <ul class="nest_menu">
      <li><a href="#">メニュー１－１</a></li>
      <li><a href="#">メニュー１－２</a></li>
      <li><a href="#">メニュー１－３</a></li>
    </ul>
  </li>
  <li><a href="#">メニュー２</a></li>
  <li><a href="#">メニュー３</a></li>
</ul>
.menu {
  width: 500px;
  margin: 0;
  padding: 16px;
  display: flex;
  background-color: #efefef;
}
.menu > li {
  margin: 0;
  padding: 0 24px;
  border-right: 1px solid black;
  list-style: none;
  color: #444;
  position: relative;
}
.menu > li > a {
  text-decoration: none;
}
.menu > li > .nest_menu {
  width: max-content;
  margin: 0;
  padding: 0;
  position: absolute;
  top: 40px;
  left: 0;
  background-color: #efefef;
  opacity: 0;
  transition: all 0.3s;
}
.menu > li:hover > .nest_menu {
  opacity: 1;
}
.menu > li > .nest_menu > li {
  margin: 0;
  padding: 12px 24px;
  box-sizing: border-box;
  overflow: hidden;
}
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