PURPOSE: To form a single-phase three-winding transformer with a tertiary winding, the range of impedance thereof, which can be selected, is wide, without increasing size, weight and manufacturing cost.

CONSTITUTION: A main leg side excitation winding 15A, a medium-voltage winding (a shunt winding) 2A and a high-voltage winding (a series winding) 1A are mounted concentrically on a main leg section 9A for a core so as to mutually keep the distances of insulation in order from the inside, and a tertiary series winding 13B, an excitation winding 15B functioning as a tertiary shunt winding in combination and a tap winding 4B are concentrically on the return path leg 9 side of the core so as to keep the required distances of insulation in order from the inside. According to such a constitution, leakage impedance between the tertiary series winding 13B and the medium-voltage winding 2A is the sum of leakage impedance ZA between the excitation winding 15A and the medium- voltage winding 2A and leakage impedance ZB between the tertiary series winding 13B and the excitation winding 15B functioning as the tertiary shunt winding in combination, and the latter impedance ZB can be determined at a predetermined value by changing the winding ratio of a tertiary winding, thus easily convering a wide impedance range.