PURPOSE: To realize reliable fail safe without influence during a normal operation by providing a spring on a plunger at its side opposite to a sliding member, which plunger is slidably arranged on one end of the sliding member, enabling both pressing and sliding of the sliding member, and forming a pressure receiving part from a compressed air source on the side of the sliding member of the plunger.

CONSTITUTION: A spool 112 which is slid by means of a stepping motor 128 is provided inside a sleeve 10 of a driving rod 108 connected to a load fixed to a piston 104 through a ball screw shaft 120 and a nut 118. A plunger 200 having a pressure receiving part 200A and a through-hole 200B is fitted to an end side of the spool 112 inside the sleeve 106, while a spring 202 is provided inbetween the sleeve 106. Accordingly, a driving force and responsiveness are not influenced during a normal operation, and when a failure occurs at a compressed air source, the driving rod 108 is retreated to a stroke end with certainty. Satisfactory cost performance and reliable operation are secured.