<div class="panel-group"> <div class="panel panel-default"> <div class="panel-heading"> <h4 class="panel-title"> <a data-toggle="collapse" href="#acc1"> 目次1 </a> </h4> </div> <div id="acc1" class="panel-collapse collapse"> <div class="panel-body"> <div class="panel-group"> <div class="panel panel-default"> <div class="panel-heading"> <h4 class="panel-title"> <a data-toggle="collapse" href="#acc11"