To eliminate dangerous horizontal installation that the horizontal installation is allowed some times even when one of jacks is not grounded, when the jacks are fully extended once to bring tires into a floated condition and then the higher-side jack is contracted to set a working vehicle horizontally.

This controller of the present invention is arranged with solenoid valves 11, 12, 13, 14 for controlling supply and discharge of pressurized oil in the every jack 6, 7, 9, 10, grounding detectors 6d, 7d, 9c, 10c for detecting the grounding of the every jack, and an inclination detector 15 for detecting inclinations in longitudinal and lateral directions of the working vehicle, and provided with a control means 20 for receiving signals from the respective detectors to output control signals to the respective solenoid valves. The control means 20 finishes the horizontal installation for the jacks when the means 20 detects the substantial horizontal of the working vehicle by the detector 15 and detects the groundings of all the jacks 6, 7, 9, 10 by the respective detectors 6SSd, 7d, 9c, 10c.