PURPOSE: To prevent overload properly by simulating temperature distribution based on the heat characteristics of a heat generating section in an electrical equipment, estimating the temperature of the heat generating section and outputting an alarm when the value of the temperature of the heat generating section exceeds a reference value.

CONSTITUTION: Regarding a heat generating section 2 in an electrical equipment monitoring a temperature rise and a constitutional section continued to the heat generating section, a heat equivalent circuit 6 simulating the temperature distribution of each section on the basis of each heat characteristic is formed by using hardware or software, Joule loss generated in the heat generating section 2 is detected successively, and signals corresponding to the Joule loss are input to the heat equivalent circuit 6. The temperature rise of the heat generating section 2 is estimated from the voltage value of the input point of the heat equivalent circuit 6, and the alarm of overload is output when the temperature rise exceeds a specified threshold value. Accordingly, the high accurate temperature rise of the heat generating section 2 of the electrical equipment as an object to be protected can be estimated, thus outputting the alarm of precise overload.