Abstract: A side airbag apparatus includes an inflator, which generates inflation gas in response to an impact applied from a side of a seat of a vehicle, an airbag, which is deployed and inflated forward with inflation gas on a side of an occupant seated in the seat, and an accommodating portion, which accommodates the airbag folded into an accommodation state. The airbag in the accommodation state is formed through first and second folding operations in a state where the airbag is deployed but not inflated. In the first folding operation, an upper portion of the airbag is folded downward. In the second folding operation, which is after the first folding operation, the airbag is folded rearward from front.