PURPOSE: To reduce a temperature in a join part of a power device by providing a means capable of sensing and corresponding to a temperature of the power device and acting on a reference voltage generator to reduce a reference voltage corresponding to a temperature rise exceeding a preset value.

CONSTITUTION: A first input 12 connected with a power device 5 and a second input 14 connected with a reference voltage generator 15 are input into a comparator 11. When an output of the comparator 11 exceeds a reference voltage, a signal generator 17 generates a diagnostic signal. A means 16 which senses a corresponds to a temperature of the power device 5 and acts on the reference voltage generator 15 so as to reduce the reference voltage corresponding to a temperature rise exceeding a set value is provided. Consequently, it is possible to reduce a temperature of a join part of the power device.