PURPOSE: To detect the state of loading accurately by comparing the counted value of a counter with the first preset value, and presetting the second preset value in the counter as an initial value in response to the output signal of an FF.

CONSTITUTION: A voltage detector 4 outputs a detected signal (e) of an AC power source 1. A current detecting circuit 5 outputs a detected current signal (i) of an induction motor 2. Polarity judging circuits 6 and 7 judge the polarities of the signals (e) and (i) and output logic signals E and I. A counter 12 counts clock pulses in specified cycles during the period of the phase difference. In a comparator 14, the counted value of the counter 12 and the first preset value are compared, and the fact that the specified correlation is achieved is detected. An FF 15 is set and reset with the output signals from the comparator 14. The second preset value is preset in the counter as an initial value in response to the output signal of the FF 15. The detecting level is set with the first preset value. The width of hysteresis is set with the second preset value. Thus, the accurate detection of the state of loading can be performed.