A side airbag apparatus, which deploys an airbag between an occupant of a vehicle and a sidewall of said vehicle in order to protect the occupant in the event of side collision of the vehicle, includes a lower air bag unit and a control unit. The lower airbag unit has a waist airbag, which deploys toward the hip of the occupant M and pushes the hip of the occupant M in the direction opposite to the sidewall, an inflator, which generates gas for inflating the waist airbag and supplies gas to the waist airbag, and a module case, which stores the waist airbag and inflator therein. The control unit ignites the inflator in order to generate gas when an acceleration of greater than a prescribed value is detected.