PURPOSE: To attain easily at a low cost the detection of an equivalent winding current by multiplying one current value at the winding of a transformer with a tap by a transformation ratio of the transformer corrected automatically by a tap position.

CONSTITUTION: In a secondary voltage mimic circuit 26, as a secondary voltage of a transformer 10, a voltage V2 adjusted at every tap by a contact 30 which is opened and closed in accordance with each tap position, and a variable resistance 32 is set so that it can be fetched by a variable resistance 34. Subsequently, this secondary voltage V2 is inputted to a correcting circuit 24 together with a voltage corresponding to a primary current of the transformer 10 from a current - voltage transducer 20, and a voltage drop by impedance % ZT of the transformer 10 is corrected. By this output voltage V2', a voltage V1 corresponding to a primary voltage from a primary voltage setting circuit 38 is divided by a divider 40, this voltage ratio V1/V2' is multiplied by an output of the current - voltage transducer 20 by a multiplier 22, and a voltage corresponding to a secondary current is obtained.