In a control system of a variable displacement compressor, a pressure sensing means mechanically detects at least one pressure in the refrigerant circuit. A varying means varies a reference value for positioning a valve body. A pressure detector electrically detects the pressure detected by the pressure sensing means in the refrigerant circuit and/or physical quantity which correlates with the detected pressure. A calculator calculates a maximum value of urging force applied to the valve body by the varying means toward an increasing side of the displacement of the compressor. The urging force applied to the valve body is controlled not to exceed the maximum value toward the increasing side of the displacement. The displacement of the compressor is maximized by the pressure sensing means under the pressure for calculating the maximum value when the varying means applies urging force of the maximum value to the valve body.