HTML
1<div class="modal-dialog-centered" role="document">
が
HTML
1<div class="modal-dialog modal-dialog-centered" role="document">
で動くかと
※挙動確認したソース
HTML
1<html>
2<head>
3<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css" integrity="sha384-MCw98/SFnGE8fJT3GXwEOngsV7Zt27NXFoaoApmYm81iuXoPkFOJwJ8ERdknLPMO" crossorigin="anonymous">
4<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
5<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js" integrity="sha384-ZMP7rVo3mIykV+2+9J3UJ46jBk0WLaUAdn689aCwoqbBJiSnjAK/l8WvCWPIPm49" crossorigin="anonymous"></script>
6<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js" integrity="sha384-ChfqqxuZUCnJSK3+MXmPNIyE6ZbWh2IMqE241rYiqJxyMiZ6OW/JmZQ5stwEULTy" crossorigin="anonymous"></script>
7</head>
8<body>
9<!-- Button trigger modal -->
10<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#deleteModalCenter">
11 Launch demo modal
12</button>
13<!--削除確認モーダル-->
14<div class="modal fade" id="deleteModalCenter" tabindex="-1" role="dialog" aria-labelledby="deleteModalCenterTitle" aria-hidden="true">
15 <div class="modal-dialog modal-dialog-centered" role="document">
16 <div class="modal-content">
17 <div class="modal-header">
18 <h5 class="modal-title" id="deleteModalCenterTitle">確認</h5>
19 </div>
20 <div class="modal-body">
21 <h2>本当に削除しますか?</h2>
22 </div>
23 <div class="modal-footer">
24 <button type="button" class="btn btn-secondary" data-dismiss="modal">閉じる</button>
25 <button type="button" class="btn btn-danger">削除する</button>
26 </div>
27 </div>
28 </div>
29</div>
30</body>
31</html>