PURPOSE: To secure both of the superior lubrication operation and the pressurizing operation of a piston having the superior responsiveness by arranging a seal for the second liquid and a seal for the pressurized working oil in separation so that each sliding range does not interfere.

CONSTITUTION: On the outside of the pressurizing chamber 20D of a working chamber 24, the second liquid different from the pressurized working oil is built in. A sealing mechanism 22 is installed between the inner wall of the working chamber 24 and the sliding part of a piston 20 so that the liquid tightness between the part where the second liquid is built in and the pressurized chamber 20D. The sealing mechanism 22 is constituted of the seals 22A and 22D for the second liquid installed on the side where the second liquid is built in and the seals 22B and 22C for the pressurized working oil which are installed on the pressurizing chamber 20D side. The seals 22A and 22D for the second liquid and the seals 22B and 22C for the pressurized working oil are set so that each sliding range does not interfere each other. Accordingly, both of the superior lubrication performance and the pressurizing operation of the piston 20 having the superior responsiveness can be secured.