To move an actuator at a constant speed.

A stair-form position signal to change a position set signal 13 by a unit amount at intervals of a unit time is outputted by a controller 18. After the position signal is compared with a feedback signal from a position detector 17, which detects the position of an edge mask 9, for computation, a computing result is amplified by a proportional amplifier 14 and converted into a flow rate by an electricity oil converter 15, and the edge mask 9 is moved by a hydraulic cylinder 10. The unit amount change of a position signal is set to a value within a change amount executed by the hydraulic cylinder 10 and control systems 14 and 15 within a unit time.