A side airbag apparatus includes an airbag 30 and an inflator assembly 20 incorporated in the airbag 30. The airbag 30 is formed by first and second inflation sections 31, 42. An insertion portion 49 in an upper portion of the second inflation section 42 is located in a lower portion of a receptacle portion 38 of the first inflation section 31. The gas discharge passages 57 for discharging inflation gas G in the first inflation section 31 to the outside of the airbag 30 are formed between the overlapped insertion portion 49 and receptacle portion 38. In this state, the first and second inflation sections 31, 42 are connected to each other. When the first inflation section 31 is not restraining the occupant, the inflation gas G in each inflation section 31, 42 causes the insertion portion 49 to closely contact the receptacle portion 38, so that the gas discharge passages 57 are closed. When the first inflation section 31 is restraining the occupant P, a part of the insertion portion 49 is separated from the receptacle portion 38 by using the increase in the internal pressure of the first inflation section 31 due to the restraint, so that the gas discharge passages 57 are opened.