To prevent a current at abnormal voltage from flowing to the transformer TR of a main circuit to cause serious accident such as burning, etc., even in case that a control circuit switches a tap, with a semiconductor switch faulty in open mode or short circuit mode.

A voltage controller 1 is composed of n pieces of taps on the primary side of the transformer TR of a main circuit, semiconductor switches SSR1-SSRn for switching these taps, and a control circuit, and the iron core 2 of the transformer TR of the main circuit has an X-phase winding and a Y-phase winding wound, and is provided with a magnetic sensor 3. In case that the balance of the magnetic flux produced by the X-phase winding and the Y-phase winding wound on the iron core of the transformer of the main circuit collapses and that leaked magnetic fluxes more than usual is produced from the iron core 2 excluding the case that different taps are selected for X phase and Y phase or that the phase on one side opens, there is a possibility of an abnormal current flowing in the transformer TR of the main circuit by the fault or the like of the semiconductor switch, so the abnormality of the circuit is transmitted to the control circuit by detecting this magnetic flux with a magnetic sensor 3, and a fault preventive measures such as switching it into direction sending or the like is performed to avoid abnormality.