<ul class="list-group">
  <li class="list-group-item" *ngFor="#todo of todos">
    <div class="row">
      <div class="col-xs-10">
        <label class="">
          <input type="checkbox"
                 (click)="toggleComplate(todo)"
                 [checked]="todo.isCompleted">
        </label>
        <span [class.complate]="todo.isCompleted">
         {{todo.id}}. {{todo.title}}
        </span>
      </div>
      <div class="col-xs-2 text-center">
        <button class="btn btn-link" (click)="deleteTodo(todo)">削除</button>
      </div>
    </div>
  </li>
</ul>

<ul class="list-group">
  <li class="list-group-item" *ngFor="let todo of todos; let i = index">
    <div class="row">
      <div class="col-xs-10">
        <label class="">
          <input type="checkbox"
                 (click)="toggleComplate(todo)"
                 [checked]="todo.isCompleted">
        </label>
        <span [class.complate]="todo.isCompleted">
         {{i + 1}}. {{todo.title}}
        </span>
      </div>
      <div class="col-xs-2 text-center">
        <button class="btn btn-link" (click)="deleteTodo(todo)">削除</button>
      </div>
    </div>
  </li>
</ul>

Change to displaying array index instead of id
:+1: Change to displaying array index instead of id
