The present invention is applied to an airbag that is divided into a first inflation chamber and a second inflation chamber by a vertical partition having a communication hole. The airbag is folded to be turned into a storage form and stored in a storage portion located in the vicinity of a side of an occupant. The airbag is turned into the storage form by being subjected to first to third folding operations. In the first folding, an upper portion of the airbag in an uninflated and deployed state is folded back away from the occupant along a folding line that is inclined to be lower toward the front end. In the second folding, the airbag is folded from the front to the rear. In the third folding, an upper portion of the airbag is folded back away from the occupant.