To make a structure compact by providing a holding means maintaining a forcibly locking condition to a cylinder wall part in a pressure intensifier provided in a fluid pressure piston front end part and moving inside the cylinder together with it.

A differential piston working as a hydraulic piston and consisting of a piston rod 3 and a fluid pressure piston 2 serves as a pressure switch or a controller and presses a second piston rod 6. When a counter directional force working on the piston rod 6 exceeds a set value, a ball 4 presses a spring 5 outward so as to turn a horizontal part in a control/regulation projection part 3b, so that a pressure in a low-pressure chamber 7 based on a spring 10 is increased according to an advancing action of the piston rod 3. Then, a fastening part 9 in a holding member 11 is pressed and returned backward slightly so as to be pressed to a cylinder 1 inside wall, and the holding member 11 works as a collet member. On the other hand, a piston 8 and the rod 6 are pushed forward by a high pressure generated accordingly. According to the backward action, the holding member 11 is released by means of an action of a stopper ring 12.