To dispense with a detecting means such as a switch or a pressure switch by changing rear wheels to non-brake at outputting the grounding signals of hydraulic jacks, simultaneously with outputting a changeover command signal from a manual operation means to a controller.

A controller 13 is provided with an operation detecting means 14 detecting a changeover command signal from a manual operation means 7 and outputting an operation detecting signal (a), and a jack grounding detecting means 16 detecting grounding of hydraulic jacks 3. By operating the manual operation means 7 for driving the hydraulic jacks 3, the contact of a relay 15 becomes ON, the hydraulic jacks 3 are grounded to float front wheels, and the contact 16a of the jack grounding detecting means 16 becomes ON. Hereby, a solenoid valve 25 is closed, air from a parking brake valve 24 to air boosters 17, 18 is intercepted, the front wheels and the rear wheels become in a non-brake state. Hereby, even if difference is generated in a distance between the grounding plates of the hydraulic jacks 3 and the rear wheels, excessive bending force does not act on the hydraulic jacks 3.