A side airbag apparatus includes an airbag main body. The inside of the airbag main body is divided by a vertical partition including a gas flow restrictor at least into an upstream inflation portion, which is located rearward of the vertical partition and receives inflating gas, and a downstream inflation portion, which is located forward of the vertical partition and receives the inflating gas that has passed through the upstream inflation portion and the gas flow restrictor. The dimension of the vertical partition in the widthwise direction of the automobile seat is set to a dimension that is equal or approximate to the dimension of the airbag main body in the widthwise direction in a case in which the airbag main body is inflated without the vertical partition.