PURPOSE: To obtain a highly accurate signal depending on load conditions such as changes in torque, in rotational speed by detecting the load conditions of an induction motor from a phase difference between coil current and applied voltage.

CONSTITUTION: By a phase difference detecting section 16, a phase difference Pd between the coil current and applied voltage of an induction motor is detected based on a difference between respective time points of zero-cross. By a counter control section 24, a counter 23 is count-worked only for a time according to the phase difference detected by the phase difference detecting section 16. From a load condition discriminating section 28, the output of load condition signal Ss according to count value by the counter 23 is generated.