A support member has a sheet-shaped body that expands and contracts and that supports a cushion, and a frame-shaped body that fixes the sheet-shaped body in a tensioned state. The sheet-shaped body is attached to a frame member by the frame-shaped body being engaged with the frame member. The frame-shaped body is rigid and maintains a shape that follows an outer shape of the frame member when an occupant is seated, and has a first portion where an end portion of the sheet-shaped body is fixed, and a second portion where the end portion of the sheet-shaped body is not fixed. The second portion is formed at a position that corresponds to a position where a load of the occupant concentrates when the occupant is seated.