PURPOSE: To perform an accurate diagnosis on abnormality on contact resistance by a method wherein, current component and generated heat are obtained and corrected by use of ambient temperature.

CONSTITUTION: A contactless temperature sensor 16 is attached to the sealed container 7 where a movable contact 11 and a fixed contact 10 are interlocked. The output from the sensor 16 is connected to the input X1 of the comparator 20 through the amplifier 17. The secondary output from the current transformer 5 is brought through the logarithmic amplifier 18 into the integrator 19, and its output is connected to the input X2 of the comparator 23. The output from the ambient temperature sensor 21 at the power switch is passed through the amplifier 22 to the inputs Y1, Y2 of the comparators 20, 23. The outputs from each of the comparators 20, 23 are brought into each input X3, Y3 of the calculation amplifier 24, respectively. The output from the calculation amplifier 24 is connected to the control circuit 25 which controls the display 26 and the recorder 27.