PURPOSE: To conduct an accurate weighing and ensure the detection of an abnormal plasticization by a method wherein a tolerance of a reference screw torque in a weighing operation is set and stored, a screw torque is detected during weighing, and an abnormality detection signal is outputted when the detected value is out of the tolerance.

CONSTITUTION: An upper tolerance limit and a lower tolerance limit are set for a value of a reference screw torque and stored in a nonvolatile memory 24 of a controller 10. A PMC CPU 18 makes a CNC CPU 25 start a drive control of a weighing process, reads a present position of a screw 2 and a present value of a drive current of a screw rotating servo motor M2 from a present position storage register of a memory 19, and finds a value of a torque. If the torque is out of the upper tolerance limit or the lower tolerance limit, an alarm message is displayed on a screen of a manual data input device with display 29, or the drive of an injection molding machine is brought to a stop.