PURPOSE: To use a single rod double action type hydraulic cylinder by forming two control valve bypass lines one end of each is connected between the single rod double action type hydraulic cylinder and a control valve and a tank or a pressure source, and including a damping valve and a check valve.

CONSTITUTION: When a hydraulic source 3 is driven by the drive of an engine for traveling a vehicle, hydraulic pressure is accumulated in an accumulator 5 and also the single rod double action type hydraulic cylinder 1 is extended and contacted via a control valve 2 to adjust a ground clearance. Meantime, when electricity supply is stopped due to the failure of an electric system fail- safe action takes place to make the control valve 2, inoperative and the extension and the contraction of the hydraulic cylinder 1 is inhibited, but since the front and the rear of the control valve 2 is connected to each other via bypass lines 6, 7, the extension and the contraction of the hydraulic cylinder 1 is allowed. In addition, the damping action of the damping valve 6a, 7a becomes available so that the single rod double action type hydraulic cylinder can be used by this construction for displaying its damping function.