SIDE AIRBAG

A side airbag installed on a side surface, which faces a passenger seat, of a seatback of a driver's seat, is provided. The side airbag includes a main cushion provided with a flow hole and expandible by gas, and a sub-cushion that is attached to the main cushion, the sub-cushion being connected to the flow hole. When the main cushion expands after the airbag is actuated, the sub-cushion protrudes from the main cushion while being expanded by the gas supplied through the flow hole, and as the main cushion expands, the main cushion is deployed toward a front side of a vehicle between the driver's seat and the passenger seat, and the sub-cushion protrudes from a rear side of the main cushion toward a rear side of the vehicle.