A seat cushion is swingable about an axis A with respect to a seat cushion support frame that is a part of a main frame. A cushion return spring that is a flat spring is disposed between the seat cushion and the seat cushion support frame so that an elastic force of the cushion return spring returns the seat cushion to a neutral position. A pair of clamp rollers that clamps the cushion return spring is disposed. The clamp rollers are movable along a longitudinal axis of the cushion return spring. Changing of a clamping position by moving the clamp rollers changes an effective length of the cushion return spring. In this way, a swingable range of the seat cushion and a force to return the seat cushion to the neutral position are changed.