PURPOSE: To improve the accuracy of a stopping position by alternately repeating the first procedure for adding a thrust and the second procedure for eliminating a thrust when the shift of a piston reaches near the stopping target position.

CONSTITUTION: Between chambers 9, 10 of an air cylinder 1 and a power source 6 are provided variable throttle valves 7, 8, while between the chambers 9, 10 and the atmosphere are provided variable throttle valves 11, 12. In the case when a piston 3 is moved rightward, and when it reaches near the stopping target position, the first procedure for closing the variable throttle valve 8, 11, widely opening the variable throttle valve 7 and slightly opening the variable throttle valve 12, and the second procedure for closing the variable throttle valves 11, 12 and widely opening the variable throttle valves 7, 8 are repeated alternately. The shift speed of the piston 3 can be extremely lowered when it reaches the stopping target position, thereby the accuracy of the stopping position is improved.