SIDE AIR BAG DEVICE

Provided is a side air bag device (S) which is disposed in a space between a vehicle body and a back seat and for which rigidity of attachment to a vehicle body member is improved. The side air bag device (S) provided between a vehicle body and a back seat includes: a base member (20) including a base storage (24) configured to store an air bag (10) and an inflator (11) and a base attachment portion (21a) attached to the vehicle body; and a retainer member (30) configured to hold the base storage (24) from a vehicle back side, The retainer member (30) extends from a portion holding the base storage (24) while being bent along the shape of the vehicle body. The retainer member (30) is attached to the vehicle body together with the base attachment portion (21a) while being overlapped with the base attachment portion 21a.