Influence from a rear bag section is prevented or suppressed from being applied to an occupant out of position. In a state in which a side airbag inflates and deploys frontward of a side support section of a seat back, a front-rear partition cloth that partitions a bag body into a front bag section and a rear bag section brings an intermediate portion in a vehicle width direction of a front surface of the rear bag section to be recessed toward a vehicle rearward direction so as to be a valley-shaped recessed portion. Through this, even if the side airbag inflates and deploys while the occupant is located frontward of the side support section, it is possible to prevent or suppress influence from the rear bag section to this occupant.