An airbag door is openably attached to an instrument panel. A cushion pad is attached inside the airbag door, and has a stepped separable pad. A cushion bracket is attached to the cushion pad. A cushion is mounted inside the cushion bracket. The cushion separates the separable pad and opens the airbag door upon inflation. A reinforcement member is attached to the cushion near a gas inflow hole in the cushion. An inflator assembly for inflating the cushion is connected to the cushion by a mounting bracket.