PURPOSE: To take a countermeasure to an accident before electric equipment breaks down thermally, by finding an internal fault in its early stage by detecting a rise in the internal temperature of the electrical equipment due to local heat generation through continuous monitoring by infrared-ray irradiation.

CONSTITUTION: If the short-circuit part 7 of a mode coil 1 causes a short circuit, the temperature of the short-circuit part 7 rises, so thermal radiation which corresponds to the temperature is performed to radiate infrared rays 8. Those infrared rays 8 are inputted to a heat-sensitive detecting element 2 to be converted into an electric temperature signal which corresponds to its intensity, thereby supplying the signal to an output part 4. The output part 4 has a set temperature signal which corresponds to the upper limit temperature of the mode coil 1 and when an input signal higher than the set temperature signal is applied from an amplification part 3, the occurrence of the internal fault is discriminated. Thus, a necessary countermeasure to an accident is taken accurately before thermal destruction due to the short circuit of the coil 1 occurs.