PURPOSE: To reduce use quantity of hydraulic fluid and to speed up operation by arranging a supply source to which high-pressure hydraulic fluid is supplied, a power piston located near the supply source, and a control valve that keeps abreast of a mechanism including a power piston.

CONSTITUTION: This actuating mechanism has a shaft 1 connected to a piston 5 in cylinder 11a. The cylinder is defined by sleeve 7 that is surrounded by a control valve 9 in the main body 3, and is connected to a high-pressure cylinder 21 through a port 17 and a low-pressure return cylinder 23 through ports 13, 19. The high-pressure fluid cylinder 21 is controlled by a control valve 9, and selectively supplies fluid to one side of the piston 5. A latch apparatus keeps abreast of an armature 43 connected to a control valve 8 to control the actuating mechanism. Furthermore, an elastic breaking means 57 increases the speed lowing force with nearing to the valve open position, and the control valve 9 returns the piston 5 to the valve close position when high-pressure is relieved.