An airbag is provided with first and second tethers and attached to the inner surface of the ceiling. The first tether extends towards the left side of the back face side from the center portion of the front face side facing a rider. The second tether extends towards the right side of the back face side from the center portion of the front face side. The first and second tethers are arranged into a substantially V-shape with the bottom on the rider's side. When the airbag is compressed in the back and forth direction by receiving a forward momentum of the rider, the expansion of the airbag is regulated by the first and second tethers on the front face side. The expansion in the right and left direction is not regulated by the first and second tethers on the back face side of the airbag. Accordingly, the airbag expands in the right and left direction on the back face side so as to have a wide width.