An airbag includes on the rear plane a passenger protection portion for protecting a front seat passenger. The passenger protection portion includes a front-collision arresting plane for protecting a head of the passenger as he moves forward in the event of a frontal collision of the vehicle and an oblique-collision arresting plane that is disposed at least either on the left or on the right of the front-collision arresting plane in such a manner as to protrude rearward out of the front-collision arresting plane for protecting the head of the passenger as he moves diagonally forward in the event of an oblique or offset collision of the vehicle. The passenger protection portion further includes an arresting recess that is disposed between the front-collision arresting plane and oblique-collision arresting plane in such a manner as to be sunken forward for receiving and arresting the head of the passenger.