PURPOSE: To greatly reduce the load loss, by reducing the range of fluctuation in % IZ of each tap voltage.

CONSTITUTION: A four-leg iron core having two main legs 1, 2 and two side legs is used. Tertiary windings 31, 32 are provided on the main legs 1, 2. Low-voltage windings 21, 22 are provided on the tertiary windings 31, 32. High-voltage windings 11, 12 are provided on the low-voltage windings 21, 22. An exciting winding 33 is provided on one side leg 3. A rough tap winding 40b is provided on the exciting winding 33 concentrically. A dense tap winding 40a fitted with a plurality of taps is provided on the rough tap winding concentrically. Thereby, the range of fluctuation in % IZ which influences the fluctuation in the short-circuit capacity or the voltage fluctuation ratio can be greatly reduced. The load loss can be also greatly reduced.