PURPOSE: To reduce the dimension and weight of a hydraulic pump and facilitate air venting-off work by installing a booster which outputs a hydraulic pressure which is amplified according to the difference of the sectional surface area of a piston onto a hydraulic actuator and installing a check valve onto the booster.

CONSTITUTION: A hydraulic actuator 77 is equipped with a booster 83 which receives the pump pressure of a hydraulic pump 101 and outputs a hydraulic pressure which is amplified by the difference of the sectional surface area of a piston. A check valve 115 is installed onto the booster 83. When air venting-off is carried out for the hydraulic actuator 77 and the booster 83, the check valve 115 is opened, and the pressurized liquid supplied form the hydraulic pump 101 can be supplied into the hydraulic actuator 77. Therefore, the small-sized and lightweight hydraulic pump can be obtained, and the air venting work can be facilitated.