A side impact airbag apparatus includes an inflator 41 that discharges inflation gas, an airbag 20 that incorporates the inflator in a rear portion and is folded to form a storage state, and a storage portion located in the vicinity of an outer side of an occupant seated on a vehicle seat. The storage portion stores the inflator 41 and the airbag 20 when in the storage state. Inflation gas discharged from the inflator 41 in response to a side collision inflates and deploys the airbag 20 into a space between a body side portion of the vehicle and the vehicle seat while unfolding the airbag 20. A front portion of the airbag 20 in a deployed state that is forward of the inflator 41 is folded from the front toward the rear, so that the airbag 20 is made into a transitional form elongated in an up-down direction. A lower portion 37 of the airbag 20 in the transitional form below the inflator 41 is folded in a bellows-like manner from the bottom toward the top such that folded sections 37A are perpendicular to the up-down direction. Accordingly, the airbag 20 is made into a storage state.