To adjust the discharge quantity of a hydraulic pump in response to the traveling condition or traveling state of a vehicle, secure the oil quantity required by a hydraulic suspension mechanism, and save the power of an engine.

This hydraulic suspension mechanism feeds a output signal of an attitude control circuit 21 based on the signals of an lateral acceleration sensor 22, a vertical acceleration sensor 23, a longitudinal acceleration sensor 24 and a car height sensor 25 to a hydraulic control valve 4d and controls the oil quantity of the oil chamber 4e of an actuator 4b. The output signal of a discharge quantity control circuit 17 based on the signals of the lateral acceleration sensor 22, vertical acceleration sensor 23, longitudinal acceleration sensor 24 and car height sensor 25 and the signals of the inclination sensor 4c of a swash plate 4a, an engine speed sensor 18 and a vehicle speed sensor 20 is fed to the hydraulic control valve 4d, and the discharge quantity of a hydraulic pump 4 is steplessly controlled in response to stages when a vehicle is traveled on a rough road or a flat road or the vehicle is stopped.