PURPOSE: To perform automatically the detection inscribed on the title by a method wherein a raw material quantity measuring switch driven by a screw is made to generate an alarm when a measured time of fixed quantity is shorter than a time required.

CONSTITUTION: A raw material 1 is fed from a hopper 2 and is molten and kneaded by a heating cylinder 3 and a screw 4, and the molten raw material is stored at the tip part of the heating cylinder 3. The screw 4 is made to go back by the back pressure of raw material and a raw material quantity measuring switch 5 operates when the screw goes back to a predetermined distance. The molten raw mterial is ejected into a metal mold 6 and is molded. When one process of molding is completed, another storage of raw material 1 is resumed. On this occasion, when the necessitated time for the measurement of raw material mentioned above is shorter than a normal time required, it is detected by an electric circuit and an alarm is generated. When the runner 8 is obstructed, the molten raw material remains in the cylinder 3, and the measured time of raw material quantity is shortened corresponding to the remained quantity.