An airbag main body is divided into a first inflation chamber, a second inflation chamber, and a third inflation chamber. The third inflation chamber is inflated beside the lumbar region of an occupant. The vertical partition has a communication hole that connects the first inflation chamber and the second inflation chamber to each other. The airbag main body has a gas passage, which extends in the up-down direction and surrounds at least the gas outlet of an inflator. The gas passage bridges the first inflation chamber and the third inflation chamber. The gas passage has an upper opening, which faces a part of the first inflation chamber that is inflated beside the shoulder region of the occupant. The gas passage also has a lower opening, which faces the third inflation chamber.