To prevent the deterioration of a traveling performance by providing changing means for changing the spring constants of front and rear suspension shaft suspension springs and detecting means for detecting the weight of a car body and reducing the spring constants of the front and rear shaft suspension springs when the car body weight is small.

In a front two-axle car having the front end portion of a vehicle supported by a front front wheel and a front rear wheel, a suspension device of a front and rear shaft 17 having front and rear wheels is composed of a main spring 25 and a sub-spring 26, and the front and rear shaft 17 is fixed to the lower end side of the main spring 25. Both ends of the sub-spring 26 are received by front and rear spring receivers 31 and 32 attached to the lower surface of a frame 11, and the spring receivers 31 and 32 are movable by air cylinders 37. When there are no occupants, compressed air is supplied to the air cylinders 37 and the spring receivers 31 and 32 are moved to non-operation positions. Thus, the spring constant of the suspension spring for suspending the front and rear shaft 17 is reduced.