Edit in JSFiddle

<div class="flex-container">
<div class="flex-item">根本太帥</div>
</div>

.flex-container {
    display: -webkit-flex;
    display: flex;
    width: 250px;
    height: 200px;
    background-color: #ccc;
}

.flex-item {
    background-color: #f36915;
    color: white;
    margin: auto;
    width: 100px;
    height: 100px;
    
    
    text-align: center;
    line-height:100px;
}