To simplify the structure of a piping system so as to reduce the cost by putting a control valve having hysteresis, in which operating pressure and release pressure are different in the connecting state between a control air spring and a pressure source.

If load applied to a rear front shaft 17 and a rear back shaft 19 is a little, the pressure of air springs 27, 34 becomes lower, and the pressure is applied to the upper surface of an auxiliary piston 51 in an auxiliary cylinder 49. At this time, the elastic restoring force of a return spring 60 is larger than the force for pressing down the auxiliary piston 51, so that a piston 50 is placed in an upper position. An input port 54 is closed by the piston 50 to open a purge port 57. When the load is increased, the internal pressure P1 is increased and the force applied to the upper surface of the auxiliary piston 51 is increased. The piston 50 is lowered with the auxiliary piston 51, to close the input port 54 and close the purge port 57.