PURPOSE: To find abnormality in early stage, by detecting the load current of an electric machinery and obtaining an allowable temperature corresponding to the load condition through an operating circuit, and producing an alarm if the temperature in the machinery is higher than the allowable temperature.

CONSTITUTION: A temperature detecting element 3 is fixed in an electric machinery 1 and the temperature (T0) in the electric machinery is detected through a temperature detecting circuit 4. Load current of the electric machinery 1 is detected through a CT 2 and fed to an operating circuit 5. The operating circuit 5 obtains an allowable temperature (T1) corresponding to load condition based on the load current. A comparison circuit 6 compares the temperature T0 with the temperature T1 and provides an alarm to an operator when the temperature difference T0-T1 exceeds over 5°C, for example, with error being considered. Since the temperature is monitored in each load stage, abnormality can be found in early stage.