The invention includes a moving step of moving a piston to a target position by supplying and discharging compressed air to and from pressure chambers on opposite sides of the piston of a pressure cylinder by driving servo valves by a controller, a clamping step of allowing a pressure member at a tip end of a piston rod to touch a workpiece softly in this target position, and a pressing force applying step of pressing the workpiece with the pressure member in a required force.