A side airbag is housed together with an inflator containing a high pressure gas as a module in a door-side side portion of a seat. The side airbag is deployed into a space between the door and an occupant sitting in the seat, on the basis of an instruction signal from an airbag control device. An occupant sensor is disposed at a position that is located in an upper portion of a seatback of the seat and shifted from a center of the seatback with respect to directions of a width thereof toward a vehicle body center. By using the occupant sensor, it is determined whether an occupant is present at a seat of a vehicle, excluding a case where an occupant having a constitution smaller than a predetermined constitution is leaning against the side vehicle body portion. The airbag control device permits operation of the side airbag if presence of an occupant is detected by the occupant sensor. The control device prohibits operation of the side airbag if absence of an occupant is detected by the occupant sensor.