PURPOSE: To protect breakdown of an element due to transient temperature rise by calculating loss of an element considering ambient temperature within every constant period.

CONSTITUTION: First, a timer is set to a determined time to start the counting operation, Next, a duty ratio D is obtained from a gate voltage when the voltage is applied, while monitoring a voltage impressed to the gate and a current value I at this time is also read. The predetermined saturated voltage value is read from the data with such current value I for arithmetic operation. The result is then accumulated during the period preset in the timer and thereby the final value P becomes a loss of the switching element. Next, the ambient temperature is read at this timing and it is judged that an abnormality has occurred when the value P is larger than the allowable loss value determined by such an ambient temperature. With this system, use of element under the derating condition can be reduced, resulting in a factor of cost-down.