PURPOSE: To substantially make the steady-state position deviation zero by a method wherein an external force fluctuation compensating control system device which compensates the external force fluctuation functioning to a piston is provided in a control system.

CONSTITUTION: A cylinder 2 is connected to a three-way guide valve for controlling position 7 and a three-guide valve for controlling external force compensation 12. The three-way guide valve for controlling external force compensation 12 consists of an acceleration detector 13, an M coefficient multiplier 14 which calculates Mx that locomotive acceleration x is multiplied by inertial mass M, an A coefficient multiplier 15 which calculates A.P1 that an pressure area of the piston A is multiplied by a pressure of the position control system P1, a derivative action calculator 16, an adder-subtractor 17, a transfer function computing element 18 which multiplies deviation from the adder-subtractor by a transfer chracteristics G(S)=β/S2, and a multiplier 19. Since this device compensates the external force functioning to a piston 1 and the position control system device can linearly control it, the piston can be inertially moved, making steady-state position deviation zero substantially.