Engaging pieces (35) for receiving a load are provided in sections of a rear frame (26) corresponding to both ends of a seat (9) in a width direction of a vehicle so as to project upward. Movably supported plates (61), which are movably supported by the engaging pieces (35), are formed at both ends of a bottom plate (51) of the seat (9) in the width direction of the vehicle, which are sections opposed to the engaging pieces (35). These movably supported plates 61 are positioned above the other sections of the bottom plate (51). Inner cushion materials (53, 54) harder than a cushion material (52) on a seat surface side are provided between these movably supported plates (61). The bottom plate (51) of the seat (9) is movably supported by the projecting pieces (35) at both the ends thereof and is never displaced in a height direction. When a weight of a rider is applied to the seat (9) as an impact load, a section between both the ends is elastically deformed to curve downward viewed from a front-rear direction of the vehicle, and an amount of downward deformation of the bottom plate (51) can be reduced. When the rider is seated, the rider sits on the inner cushion materials (53, 54) via the cushion material (52) on a seat surface side, and an impact, which cannot be eased by the cushion material (52) on the seat surface side, is eased by the inner cushion materials (53, 54).