PURPOSE: To make an accurate decision whether a regenerative resistor for dissipating energy regenerated from a motor is overheated.

CONSTITUTION: In conventional art, an integrator 8 integrates the time when the regenerated voltage from a motor 6 exceeds a predetermined time (i.e., the time when a signal S1 becomes active) and thus integrated time is compared with a predetermined value in order to make a decision whether a regenerative resistor 2 is overheated. In the invention, a temperature detector 9 is disposed closely to the regenerative resistor 2 and a signal from the temperature detector 9 is added to the output from the integrator 8 to produce a signal S4 for making decision of overheated state. According to the invention, an accurate decision can be made while taking the ambient temperature into account.