Objective: To provide a side airbag apparatus that can protect a head portion and chest portion by an appropriate restraining force, while favorably inflating and expanding even a cushion that protects a head portion and chest portion with a small inflator, and that can ensure high OOP performance.
Resolution Means: A configuration of a side airbag apparatus 100 according to the present invention includes: a bag-like cushion 110; an inflator 150; a first demarcating part 130a that demarcates the cushion 110 into a first chamber 132 on a vehicle rearward side and a second chamber 134 on a vehicle forward side, a second demarcating part 130b that demarcates a third chamber 136 on an upper side of the first chamber 132 and second chamber 134 of the cushion 110, a first inner vent 142 disposed above the first demarcating part 130, and a second inner vent 144 disposed above the second demarcating part 130b, which flows a gas flowing into the second chamber into the third chamber, and which is provided with a check valve structure that prevents a reverse flow of the gas.