To suppress the longitudinal movement of a seat using a link without causing any impact due toa a change in load.

A seat 17 covering the upper parts of a storage box 15 and a fuel tank 16 in a freely openable-closeable manner is made as a double seat, the front end thereof is fitted to the front end of the storage box 15 with a hinge 18, and the rear end thereof is locked to the seat lock 20 provided at the rear end of a seat rail 8 with a locking fitting 19. The hinge 18 includes a vehicle side hinge member 40, a seat side hinge member 41 and link member 42, the vehicle side hinge member 40 is fitted to the storage box 15, and the link member 42 is fitted to the seat 17 and the vehicle side hinge member 40 is connected to the link member 42 with the seat side hinge member 41. The body part 42a of the link member 42 is connected to the standing wall 40b of the vehicle side hinge member 40 with a bolt 43, the upper shaft part 42b of the link member 42 is connected to the hinge arm 41b of a seat side hinge member 41 with a bolt 44, and the bolt 43 and the bolt 44 are so arranged as to be vertically overlapped with each other.