An airbag device includes a steering wheel a rim portion of which has a longer length in a left and right direction than in a front and back direction, and an airbag mounted on a boss section of the steering wheel. An outer shell of the airbag includes a vehicle-side wall that is configured to be supported by an upper surface of the rim portion when deployed, and a driver-side wall a central region of which is configured to deploy generally vertically for receiving the driver. The airbag as deployed includes, in a rear portion of the vehicle-side wall, a locking inflatable portion that protrudes downward from a vicinity of the upper surface of the rim portion of the steering wheel to be locked by a rear surface of the rim portion and prevent a front portion of the airbag from slipping forward.