To perform stable servo control by fixing a servo gain corresponding to the position of the piston of a cylinder.

A variable amplifier 20 for regulating the servo gain is provided between a control circuit 5 and a solenoid valve 3. The gain of the amplifier 20 is continuously decreased with the approach of a piston 1 from the center of a cylinder 2 to the terminal, and the gain is found corresponding to a target position x0 to stop the piston 1. The position servo gain, speed servo gain and acceleration servo gain at the time of locating the piston 1 at the center of the cylinder 2 are predetermined and based on the respective servo gains, a position signal, speed signal and acceleration signal at a current position (x) of the piston 1 are found and added. Then, a control signal is prepared by multiplying the gain of the amplifier 20 by this added signal, this control signal is outputted to the solenoid valve 3, and the piston 1 is stopped at the target position x0.