PURPOSE: To measure the turn ratio with high accuracy without being affected by the coupling degree of two coils by measuring each terminal voltages when a voltage is applied from a primary winding side and when a voltage is applied from a secondary winding side.

CONSTITUTION: A switch which connects an oscillator OSC and voltmeters V1 and V2 to the primary winding and secondary winding selectively is switched under the control of a control computing element to apply the output of the oscillator OSC to the primary winding first and the terminal voltages across the primary winding and secondary winding are measured by the voltmeters V1 and V2 to obtain K1 as their ratio V2/V1. Then the switch is switched to apply the output of the oscillator OSC to the secondary winding, and the terminal voltages are measured by the voltmeters V1 and V2 as well to find K2 as their ratio V1/V2. Then (K1/K2)1/2 is found as the turn ratio which is not affected by a gap. Namely, the turn ratio is found from n2/n1=(K1/K2)1/2.