PURPOSE: To keep a seat horizontal to stabilize a seated position for improved riding comfort even on an irregular sharp gradient place by operating a seat inclining mechanism according to a mileage measured from a fixed point by a mileage measuring portion, thus keeping the seat horizontal.

CONSTITUTION: An electromagnetic brake 29 is mounted on the outside end of a drive shaft 25 and a counter 32 serving as a mileage measuring portion 31 is mounted between a bearing portion 24 and the driven sprocket 30 of a chain transmission mechanism. A seat inclining mechanism 67 uses either a rod 9 that is operated to move back and forth by the reciprocation of a decelerating motor by engagement of a pinion connected to the decelerating motor in a main body 68 with a rack formed in the rod 69, or a hydraulic cylinder. The position of a seat 11 can be held horizontal by operating the seat inclining mechanism 67 according to the gradient of a rail.