To reduce cost and to lower failure rate.

This power source circuit is provided with a power source transformer 71, a tap changing circuit 72 for changing the ratio of the number of turns of a primary winding 711 of the transformer 71 to that of the secondary winding 712, a current sensor 74 to detect the load current of the transformer 71 and a control part 75 for controlling the tap changing circuit 72 so that the load current is controlled to a specified value in accordance with the result detected by the current sensor 74. Further, the tap changing circuit 72 is provided with a reference lead-out line T0 of the primary winding 711, plural taps T1, T2, T3 and T4 led out at the different positions of the primary winding 711 and solid-state relays 77, 78, 79 and 80 connected to the taps T1, T2, T3 and T4.