To provide a voltage adjusting device which is capable of quickly switching taps and quickly adjusting voltage, even in large voltage variations.

In the voltage adjusting device connecting a secondary winding 4b of a series transformer 4 connected to power supply lines 1, 2 in series with a tapped secondary winding 5b of an exciting transformer connected between the power supply lines 1, 2 in parallel via plural AC switches 11 to 16 and connecting a bypass circuit 6 to these AC switches 11 to 16 in parallel, voltage detectors 31 to 36 are connected to both the ends of the AC switches 11 to 16 and a tap selector 23 for outputting a switching signal to the AC switches 12, 16 to be selected next, when detected voltage is more than a prescribed voltage value is connected to the voltage detectors 31 to 36.