To prevent liquid leakage from an auxiliary cylinder when a boom is stored.

When a boom storing completion advance announcement signal is input, a controller 70 opens a servo valve 74 to the oil tank 76 side. The working oil in a first liquid chamber 66b of a pressure intensifier 66 is pushed out by a piston 66a to be discharged to an oil tank 76. According to a signal from a position sensor 68, it is known that the piston 66a reaches a bottom 66d, whereby a servo valve 74 is closed, and an unload valve 84 is turned OFF. Thus, the working oil in a liquid chamber of an accumulator 86 is discharged to the oil tank 76. According to a pressure sensor 88, it is known that the working oil in the accumulator 86 is entirely discharged. Thus, at the time of storing a boom, the working oil is removed from the pressure intensifier 66.