This invention provides a method which makes it possible to reliably detect malfunctions during the ejecting step without rendering the injection molding machine more complicated. An ejector pin is incorporated in the movable die. This ejector pin is connected, via an ejector plate, a connecting rod, a connecting plate and a feed screw, with a servo motor. The force for ejecting the ejector pin pushing a molded product is measured from the driving torque of a servo motor. In this invention, the relationship between driving torque and elapsed time from the initiation of ejection of the molded product is stored as a reference pattern. The tolerance limit and monitoring interval of driving torque are preset relative to the reference pattern. When the value of driving torque falls outside the tolerance limit during the ejecting step, it is judged as a malfunction, and an alarm is generated to stop the machine in a safe state.