PURPOSE: To improve the accuracy of monitoring and to reduce the cost therefor, by digitizing an oil-hydraulic pressure inside an injection cylinder, a resin pressure inside a mold, a mold temperature and an injection ram speed, etc. through a voltage-frequency converter, and by integrating values thus obtained as integral values through a prescribed interval.

CONSTITUTION: The present method is designed for monitoring physical values of the pressure, temperature and speed, etc. which affect qualities of molded articles of an injection molding machine. An oil-hydraulic pressure through an interval (a) of a curve of oil-hydraulic pressure inside an injection cylinder in an injection-metering process, namely, through a time period t1 beginning with a time point A, for instance, is detected by a sensor 1, converted into an electric signal proportional thereto by a data input unit 2, and outputted as a signal proportional to the oil-hydraulic pressure from a V/F converter 3. The oil-hydraulic pressure through the time period t1 from the time point A of starting injection is counted and integrated by a counter 7, an integral value thus obtained is compared with a reference value by a comparator unit 9, and the presence of an abnormality, if any, is outputted from an abnormality signal generator 10.