PURPOSE: To easily and certainly inspect the quality of the winding connection in a three-phase rotary electric machinery by low voltage.

CONSTITUTION: The respective end parts considered to be on the side of a neutral point of windings 1, 2, 3 to be measured are temporarily subjected to star-connection 4. An AC power supply 5 is connected, for example, to the windings 1, 2 to be measured using measuring connection 10 to measure the synthetic impedance between the windings 1, 2 to be measured. The same measurement is also performed between the windings 2, 3 to be measured and between the windings 3, 1 to be measured and the measured impedances are compared. Since the synthetic impedance containing the winding 1, 2 or 3 of inverse connection in polarity is lowered by about half in its value as compared with the synthetic impedance in the case of regular connection, the quality of connection can be certainly inspected.