In a vehicle to which a side airbag apparatus is applied, a rear frame portion of a seat frame is arranged near a rear end of an intermediate portion in a seat back, and an intermediate pad portion of a seat pad is arranged near a front end of the intermediate portion. An airbag is provided with a main airbag and an auxiliary airbag. The main airbag is inflated between a body side portion of the vehicle and an occupant by shooting out forward from an outer side portion of the seat back by a gas jetted out of an inflator. The auxiliary airbag presses a position near the outer side portion of the intermediate pad portion forward by being inflated between the rear frame portion and the intermediate pad portion by the gas jetted out of the inflator.