An airbag includes, in a rear portion thereof, a passenger protection region for protecting a passenger. The passenger protection region includes a front-collision arresting plane, an oblique-collision arresting plane that is disposed on a left side and/or right side of the front-collision arresting plane, and an arresting recess that is disposed between the front-collision arresting plane and the oblique-collision arresting plane. Further, the airbag internally includes a tether that joints a front end portion of the airbag and the bottom of the arresting recess, and an auxiliary tether that joints the front end portion of the airbag and a side edge region of the front-collision arresting plane apart from the arresting recess. The tether and the auxiliary tether are designed to be deployed generally symmetrically with respect to a line that runs in a front and rear direction through a mounting center of the airbag to a case of the airbag.