PURPOSE: To smoothly expand a stabilizer cylinder by connecting the pilot control oil passage of a hydraulic piston to the connecting oil passage of the operating valve of a parking brake, supplying pilot hydraulic pressure from the connecting oil passage of the parking brake and exhausting it to the connecting oil passage.

CONSTITUTION: A closed oil passage 31 is connected to the bottom side chamber and the rod side wall of a stabilizer cylinder C3 and further connected to valves 32, 33 and hydraulic pistons 33, 33 mounted in a valve case 30. An oil feeding passage 34 for supplementing oil to the stabilizer cylinder C3 and the pilot control oil passage 35 of the valves 32 are attached to the valve case 30. The oil feeding passage 34 is connected to a negative parking brake B and the connecting oil passage 14 of its operating valve, thereby a change over means for the expansible state and the state incapable of expanding of the stabilizer cylinder C3 being constituted. According to this constitution, hydraulic pressure set to high pressure can be stably supplied, the stabilizer cylinder can be expanded smoothly.