PURPOSE: To simplify tilt operation by providing two check valves directed in the opposite direction to each other to be changed over from open to close position and vice versa in a bypass path affording communication between two chambers of a cylinder, and a gas chamber communication to said bypass path.

CONSTITUTION: First and second chambers 23, 24B in a cylinder 20 communicate to each other through a bypass path 32 bypassing a main piston 22. In the bypass path 32 are interposed in series a first check valve 33 allowing working oil to flow from the second chamber 24B to the first chamber 23 and a second check valve 34 directed in the opposite direction to the first check valve 33. Both check valves 33, 34 can be alternatively changed over from open to close position or vice versa by the operation of a change-over lever 35. Also, a gas chamber 40 communicates to the bypass path 32 between both check valves 33, 34 through a communicating path 39 to compensate for the increase or decrease of volume in the cylinder 20 caused by the movement of the piston 22. Further, a free piston 25 is provided in the cylinder 20, and in the respective pistons 22, 25 are provided an absorber valve 28, a return valve 29 and a relief valve 31.