PURPOSE: To decrease sliding resistance and to improve the operating speed and responsiveness by providing a labyrinth groove in the outer peripheral surface of a piston and applying plastic coating containing fluororesin to the outer peripheral surface.

CONSTITUTION: A labyrinth groove 2a extended in the direction perpendicularly intersecting the moving direction is disposed in the outer peripheral surface of a piston 2, and plastic coating 7 containing fluororesin having small frictional resistance is applied to the outer peripheral surface. Accordingly, the sliding resistance can be decreased and the operating speed and the responsive speed can be increased so as to control pressing force with high accuracy.