A clamping system provided with a fixed die plate, a movable die plate, a plurality of tie bars, half nuts able to be coupled with or release coupled parts of a plurality of tie bars, clamping cylinders accommodating pistons of a plurality of tie bars, position sensors detecting positions of pistons, a plurality of direction control valves connected to a plurality of clamping cylinders, and a control unit controlling the plurality of direction control valves so that a difference of movement amounts of pistons among the plurality of tie bars from the time of die contact to the time of the completion of clamping becomes within a predetermined permissible value based on detection results of the position sensors.