PURPOSE: To contrive to enhance responsibility by providing a pilot valve blowing air into a cylinder, and thereby controlling air actuating the pilot valve by means of a solenoid valve.

CONSTITUTION: A passage 6 connecting a cylinder 4 with a high pressure air tank 5 is opened/closed by a pilot valve 7 so as to let air be blown into the cylinder. Air actuating the pilot valve 7 in opening/closing is controlled by a solenoid valve 16. When the solenoid valve 16 is energized, a ball valve 18 allows a branching passage to be closed so as to let a passage 19 be communicated with an exhaust hole 20 for letting the lower surface of a power piston 12 be in atmospheric pressure. Which situation causes the air pressure above the upper surface of the power piston 12 to overcome spring 11 force so that a valve 10 is pressed down. Owing to this constitution, responsibility for air blowing can be enhanced.