A screw is incorporated into a heating barrel. A ball screw is connected to a rear end portion of the screw through a load cell, and a servomotor for injection is connected to the ball screw. First, the heating barrel is retreated and separated from a mold, and the screw is moved to the half stroke position. After the screw is advanced for a preset period of time, the servomotor is stopped by automatic decompression to detect an output of the load cell. Similarly, after the screw is retreated for a preset period of time, the servomotor is stopped by automatic decompression to detect an output of the load cell. An average of outputs of the load cell after the screw advances and retreats is calculated and considered to be an output of the load cell under no-load conditions, thereby performing zero adjustment of the load cell.