Suck back is started and a gradient of pressure (an amount of change in resin pressure while a screw or a plunger moves backward by a certain amount) is detected. Whether or not an absolute value of the detected gradient of the pressure is smaller than a reference value set in advance is determined, and it is determined that the reduction of the resin pressure has been completed when the absolute value becomes smaller than the reference value. A screw position at the time of the completion of the pressure reduction is stored and a screw movement amount from a suck back start position to a pressure reduction completion position is displayed on a screen.