An inspecting ECU 4 and a GD substrate 3 are connected to an inverter circuit 100 for converting a power to be supplied from a voltage source 2 and outputting the converted power to a motor 5, and switch a conducting state of each of the elements in the inverter circuit 100. A conducting state of the element is switched in accordance with a driving condition for inspection corresponding to a whole active region of the element in a state that an inspecting load 50 is connected to the inverter circuit 100. A phase current flowing in the element and a voltage between terminals of the element through a voltage detector 11 and a current detector 12 are detected to obtain a correlation of an ON time of the element and a phase current value based on the current and the voltage which are detected.