To expand a degree of freedom for setting of valve opening pressure of a hydraulic circuit while synchronizing operation of a pair of cylinder devices by connecting an upper chamber of one cylinder device and a lower chamber of the other cylinder device to each other by piping, and uniformly setting each pressure receiving areas of the upper and lower chambers.

In a device wherein a tail gate 11 is lifted up and down by means of expansion and contraction of piston rods 24, 25 of a pair of cylinder devices 17, 18, a lower chamber 33 of the cylinder device 17 and an upper chamber 36 of the cylinder device 18 are connected to a driving unit 21 through pipings 38, 39. An upper chamber 32 of the cylinder device 17 and a lower chamber 37 of the cylinder device 18 are connected to each other through a piping 40, and an upper chamber side pressure receiving area of the cylinder device 17 and a lower chamber side pressure receiving area of the cylinder device 18 are uniformly set. When operating oil in the piping 40 is shortened by thermal contraction caused by reduction of an oil temperature leakage, and the like of operating oil, operating oil is replenished by opening an automatic charge valve 68, and synchronizing operation of both cylinder devices 17, 18 is guranteed.