A movable unit in an injection molding machine is started to operate and the current physical quantity and the current time (or current position of the movable unit) are detected. The current physical quantity is stored as a reference physical quantity in association with the elapsed time of operation of the movable unit (or position of the movable unit). A deviation of the current physical quantity from the reference physical quantity is calculated and stored, a distribution index value is then calculated from the physical quantity deviations in the first to n-th cycles, and a threshold value is determined from the distribution index value. When the deviation exceeds the threshold value, alarm processing is performed.