A side airbag may be smoothly deployed when there is a side collision, and damage to the side airbag at edges of a module case is prevented or suppressed. A recess portion is formed in a seat frame made of resin, and an airbag module is accommodated in the recess portion. The airbag module is provided with a module case in a substantial box shape. An upper side gap is formed between an upper wall portion and an upper end of a side airbag, and a lower side gap is formed between a lower wall portion and a lower end of the side airbag. A length of the upper wall portion is set shorter than a length of an upper wall portion of the recess portion. As a result, the side airbag is less likely to come into contact with the upper wall portion of the module case.