To improve the leakage inductance, the fluctuation of the primary and secondary resistance values, and error of an inverter by making no-load tests, DC tests, and single-phase tests in a constant measuring means independent and providing a voltage command selector which selects the voltage commands of a single-phase testing means, a DC testing means, and a no-load testing means.

No-load tests, DC tests, and single-phase tests in a constant measuring means are made to be independent and the voltage commands of a DC testing means 6A, a no-load testing means 6B, and a single-phase testing means 6C are selected by means of a voltage command selector 8. When the selector 8 selects the DC testing means 6A, the means 6A inputs the voltage and current detected by means of a voltage detector 4 and a current detector 3, finds a primary resistance value from the ratio between the detected voltage and current, and outputs the primary resistance to a setting storing means 9. The storing means 9 stores or again stores the primary resistance value in a memory. Therefore, remeasurement can be performed easily and the leakage inductance and control error of an inverter caused by the fluctuation and errors in the secondary and primary resistance values can be improved.