A side airbag device provided at a seat back of a seat, comprises an airbag including a chest-abdomen protection portion to protect a chest and abdomen of an occupant seated and a waist protection portion to protect a waist of the occupant, and an inflator to supply gas to the airbag for an inflation at a vehicle side crash or a vehicle turnover, wherein the portions are partitioned by a partition portion that at least extends substantially rearward from an front edge of the airbag, and a recess is provided at an upper portion of the chest-abdomen protection portion. Accordingly, a whole part of side body including the chest, abdomen and waist can be properly protected regardless of the body size of the occupant, even for the occupant having the relatively small body size.