An airbag apparatus mountable on a vehicle having a side window, a first pillar adjoining the window and a second pillar adjoining the window on the opposite side is disclosed. The airbag apparatus includes an airbag adapted to be housed in an upper edge of the window, and the airbag includes a shutter section that covers the window. The shutter section includes mounting portions that are located above the first and second pillars, an inflatable region comprised of a first pillar side region, a supporting region and a preventing region, and a thin panel region. The thin panel region includes therein a catching point that is intended to catch a vehicle occupant during a rollover event of the vehicle, and the supporting inflatable region and preventing inflatable region surround the catching point continuously at the lower side and on a side of the second pillar.