PURPOSE: To enable fine adjustment and coarse adjustment to be made easily by providing a tap for excitation winding of transformer in series connection and by switching no-voltage tape switch when charging secondary voltage adjustment step voltage.

CONSTITUTION: If taps TA1 and TA2 are provided at a series transformer excitation winding 4 and selection is enabled using a no-voltage tap switch 22, a status where the tap position TA1 can be selected or the number of windings of an excitation winding 4 can be set to NET<NE. The step voltages VS' in this case is VS'=NLS×ET/NET. Since NET is smaller than Ne, the VS' becomes larger than VS and step voltage can be made larger by switching the tap from the TA2 to TA1. Thus, the secondary voltage can be adjusted finely with fine step voltage by connecting the tap of an excitation winding 4 to the TA2 within the range where the secondary voltage is high and the secondary voltage can be adjusted coarsely with coarse step voltage by connecting the tap of the excitation winding 4 to the TA1 within the range where the secondary voltage is low.