A side airbag apparatus is employed in a vehicle seat that includes a seat back and a seat belt guide. The seat back includes an accommodation portion in a lateral portion. The seat belt guide is attached to the lateral portion at a location above the accommodation portion. The side airbag apparatus includes a gas generator, an airbag, and a pressure receiving member. The gas generator and the airbag are installed in the accommodation portion. The pressure receiving member is arranged rearward of the airbag, and includes a main body and an upper restricting portion. The main body includes a pressure receiving surface, which extends along an axis of the gas generator. The upper restricting portion is located above and adjacent to the main body and includes an upper restricting surface located forward of an imaginary upward extension of the pressure receiving surface.