PURPOSE: To perform precisely a quality decision of a molded product, by a method wherein it is judged whether an action of an injection molding machine is normal or not by detecting a position where forward movement becomes impossible at the time when a pressurizing member is moved forward within a cylinder by pressurizing the pressure member by the first pressure and a position where the pressurizing member is stopped by moving the pressurizing member backward when the pressure is taken as the second pressure lower than the first pressure.

CONSTITUTION: When a screw 9 becomes unable to move forward by controlling an injection cylinder 13 by the first injection pressure P=70%, a position of the tip 9a' of a screw head is detected by an encoder 15 by taking the position as the first position and a detected value x is obtained. Then when pressure is taken as the second pressure P=20%, the screw 9 is moved backward, a position of the tip 9a of the screw head is detected by the encoder 15 by taking the position as the second position and a detected value y is obtained. Then the absolute value x-y is obtained through the first and second positions x, y supplied to a controller 4 from the encoder by a judging device 16. The value is compared with an allowable range stored beforehand in the judging device 16 and an excellent article and it is judged whether an injection molding process is normal or an abnormal action.