PURPOSE: To enable stable positioning by computing a volume inside a cylinder from a cylinder inner diameter, cylinder stroke and each measured value of pressure changes, varying a total effective sectional area of a plurality of solenoid valves according to the computed result, and thereby keeping pressure in the cylinder constant.

CONSTITUTION: In a magnetic rodless pneumatic cylinder 1, magnets are assembled in a piston and a cylinder movable part outside the cylinder. The cylinder movable part that is, a slider 3 is moved together with the movement of the piston through the magnetic connection of both magnets. The piston is moved and controlled by opening/closing a supplying solenoid valve 31 and a plurality of exhausting solenoid valves 32(32a to 32d) parallelly connected to each other, arranged in a solenoid valve box 29. A volume inside the cylinder chamber is computed based on each measured value of pneumatic pressures and a cylinder inner diameter. A total effective sectional area is varied by controlling the exhausting solenoid valves 32 having effective sectional areas proportional to the cylinder volume. Pressure in the cylinder is thus kept constant.