To lower of a seat while adapting a structure for mounting a seat lock device on a housing box.

The seat lock device 36 is mounted on a rear side projecting part 26 of a rear end part of the housing box 21. The rear side projecting part 26 is mounted on a second cross member 16 of a vehicle body frame 7. The seat lock device 36 is mounted on a center part of a vehicle width direction of the rear side projecting part 26, and a seating surface 38 supported by the second cross member 16 is formed on both sides of the seat lock device 36. The seat lock device 36 is arranged in a same position of the second cross member 16 in front and back directions of the vehicle body, and is inserted in a through-hole 48 of the second cross member 16 in a freely fitted condition.