A side airbag apparatus mountable to a seat of a vehicle, on which an occupant is seated, the side airbag apparatus comprising: a gas generating source, which generates inflation gas in response to an impact applied from a side of the seat; an airbag, which is deployed and inflated forward by the inflation gas on a side of the occupant seated in the seat; and an accommodating portion, which is located in the vicinity of a side of the occupant and accommodates the airbag folded into an accommodation state, wherein the airbag in the accommodation state is formed by folding the airbag deployed but not inflated first by a first folding operation, in which an upper portion of the airbag is folded downward, and subsequently by a second folding operation, in which the airbag is folded rearward from front, the side airbag apparatus characterized in that the airbag includes an inflatable portion, which is inflated by the inflation gas, and a partitioning member, which divides the inflatable portion into an upstream section, to which inflation gas is supplied, and a downstream section located in front of the upstream section and adjacent to the upstream section, the partitioning member includes a valve for selectively allowing and restricting flow of the inflation gas from the upstream section to the downstream section in accordance with a state where the occupant is restrained by the upstream section or in accordance with an internal pressure of the upstream section, the upstream section and the downstream section are folded by the first folding operation, the second folding operation includes an accordion-folding and a roll-folding, in the accordion-folding, the upstream section is folded such that folding directions are changed alternately, and in the roll-folding, the downstream section is repeatedly folded in the same direction. a gas generating source, which generates inflation gas in response to an impact applied from a side of the seat; an airbag, which is deployed and inflated forward by the inflation gas on a side of the occupant seated in the seat; and an accommodating portion, which is located in the vicinity of a side of the occupant and accommodates the airbag folded into an accommodation state, wherein the airbag in the accommodation state is formed by folding the airbag deployed but not inflated first by a first folding operation, in which an upper portion of the airbag is folded downward, and subsequently by a second folding operation, in which the airbag is folded rearward from front, the side airbag apparatus characterized in that the airbag includes an inflatable portion, which is inflated by the inflation gas, and a partitioning member, which divides the inflatable portion into an upstream section, to which inflation gas is supplied, and a downstream section located in front of the upstream section and adjacent to the upstream section, the partitioning member includes a valve for selectively allowing and restricting flow of the inflation gas from the upstream section to the downstream section in accordance with a state where the occupant is restrained by the upstream section or in accordance with an internal pressure of the upstream section, the upstream section and the downstream section are folded by the first folding operation, the second folding operation includes an accordion-folding and a roll-folding, in the accordion-folding, the upstream section is folded such that folding directions are changed alternately, and in the roll-folding, the downstream section is repeatedly folded in the same direction.