PURPOSE: To detect abnormality such as deterioration of an equipment such as a crusher in the elapse of time by accumulatingly adding excess current when the current value exceeds a set value as required and by issuing an overload warning when the accumulatively added value exceeds a predetermined value.

CONSTITUTION: Load current of a motor M is read out by a current detector 1 and through a current/voltage converter 2 and an A/D converter 3 it is read into a microcomputer 4 for processing. It is then checked whether or not the voltage value VD converted from the current value exceeds the predetermined voltage V1. If the predetermined voltage V1 is exceeded, then the value of (VD-V1)2t is estimated, where the sampling interval is t. If this estimated value exceeds the predetermined value VS, then a warning is issued.