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 supplies inflation gas in response to an impact applied from a side of the seat; and an airbag having an inflatable portion, which is deployed and inflated forward by the inflation gas on a side of the occupant seated in the seat to restrain the occupant, and a partitioning member, which divides the inflatable portion into an upstream section into which the inflation gas is supplied from the gas generating source and a downstream section located in front of and adjacent to the upstream section, the side airbag apparatus characterized in that the partitioning member includes a valve capable of selectively allowing and restricting flow of the inflation gas from the upstream section to the downstream section, the partitioning member having an opening and a pair of valve body elements (73, 74), wherein the partitioning member extends in an up-down direction and is located in the vicinity of the middle of the occupant in a front-rear direction when the inflatable portion has been deployed and inflated, in an initial stage of a gas feeding period during which the inflation gas is supplied, the partitioning member receives tension in the up-down direction and the transverse direction, and the pair of valve body elements (73, 74) are pushed to remain in contact with each other by the inflation gas fed into the upstream section to restrict the flow of the inflation gas from the upstream section to the downstream section through the opening, in a state in which the upstream section has inflated and restrained the occupant, the pair of valve body elements (73, 74) are separated from each other to allow the flow of the inflation gas through the opening in the partitioning member when the partitioning member is caused to warp by an external force exerted from the occupant as a result of restraining the occupant, and the upstream section has a shoulder-protecting portion, which is deployed and inflated on a side of a region of the occupant including at least a rear end part and a middle part of a shoulder of the occupant, wherein the direction in which the vehicle advances forward is referred to as the front, the reverse direction is referred to as the rear, and the up-down direction refers to the up-down direction of the vehicle. a gas generating source, which supplies inflation gas in response to an impact applied from a side of the seat; and an airbag having an inflatable portion, which is deployed and inflated forward by the inflation gas on a side of the occupant seated in the seat to restrain the occupant, and a partitioning member, which divides the inflatable portion into an upstream section into which the inflation gas is supplied from the gas generating source and a downstream section located in front of and adjacent to the upstream section, the side airbag apparatus characterized in that the partitioning member includes a valve capable of selectively allowing and restricting flow of the inflation gas from the upstream section to the downstream section, the partitioning member having an opening and a pair of valve body elements (73, 74), wherein the partitioning member extends in an up-down direction and is located in the vicinity of the middle of the occupant in a front-rear direction when the inflatable portion has been deployed and inflated, in an initial stage of a gas feeding period during which the inflation gas is supplied, the partitioning member receives tension in the up-down direction and the transverse direction, and the pair of valve body elements (73, 74) are pushed to remain in contact with each other by the inflation gas fed into the upstream section to restrict the flow of the inflation gas from the upstream section to the downstream section through the opening, in a state in which the upstream section has inflated and restrained the occupant, the pair of valve body elements (73, 74) are separated from each other to allow the flow of the inflation gas through the opening in the partitioning member when the partitioning member is caused to warp by an external force exerted from the occupant as a result of restraining the occupant, and the upstream section has a shoulder-protecting portion, which is deployed and inflated on a side of a region of the occupant including at least a rear end part and a middle part of a shoulder of the occupant, wherein the direction in which the vehicle advances forward is referred to as the front, the reverse direction is referred to as the rear, and the up-down direction refers to the up-down direction of the vehicle.