PURPOSE: To reduce the diameter and weight of a hydraulic pump and prevent oil leak and breakage by installing a check valve onto the piston member of a booster and installing a projection part which slightly projects at the top edge of the check valve.

CONSTITUTION: A booster 83 which outputs an increased pressure into a hydraulic actuator through a piston member 87 by using the pump pressure of a hydraulic pump is installed. A check valve 115 is installed onto the booster 83. A projection part 115a which slightly projects from the piston member 87 to the hydraulic pump side is installed at the top edge of the check valve 115. When the pressure of the working oil on the hydraulic actuator side increases, the projection part 115a collides with a cylinder side, and the check valve 115 is opened. Thus, the working oil pressure on the hydraulic actuator side can be lowered, and the oil leak and breakage of the hydraulic actuator can be prevented.