PURPOSE: To effectively prenotify the arrival of a screw or a cylinder at a using limit due to wear by measuring the number of rotations and rotating time of the screw at each shooting during production, and generating an alarm when the total number of rotations of the screw arrives at the number corre sponding to its life to be prenotified.

CONSTITUTION: The real number of rotations of a screw 11 is calculated by a rotation calculator 15 as N/60×t rotations according to data obtained by a sensor 15, i.e., the number of rotations of the screw N (rpm) and its rotating time(t) (sec) at each shooting. The calculated value is separately input to screw calculating means 22A and cylinder calculating means 22B of a life prenotifying device 20. The means 22A, 22B further integrate the real number of rotations of the screw at each shooting calculated by a rotation calculator 16, the integrat ed value is sequentially compared with a screw set value and a cylinder set value for prenotifying its life by comparing means, and a coincidence signal is output to notifying means 23 when any one or both coincide. The means 23 starts a predetermined notifying operation by the coincidence signal.