To keep a frame and various objects mounted on it at a horizontal state by controlling the stroke of hydraulic cylinders suspending wheels with a control device based on the signals from height sensors and a clinometer.

The rear section of the frame 2 of a crane vehicle 1 is supported by the left hydraulic cylinder 4 and right hydraulic cylinder 5 of a rear axle. A road surface 21 is inclined by an angle α against the horizontal plane, and the left hydraulic cylinder 4 is extended and the right hydraulic cylinder 5 is shrunk to keep the frame 2 nearly horizontal. Height sensors 31 and a clinometer 29 are fitted to the frame 2 and are connected to a control device 35 by electric circuits respectively. The hydraulic cylinders 4, 5 are extended or shrunk for a horizontality adjustment to keep the frame 2 of the vehicle 1 horizontal based on the predetermined reference by receiving the signals from the height sensors 31 and clinometer 29.