PURPOSE: To minimize power loss and influence to other equipment and to reduce operation work, by a method wherein the shifts of taps are detected by comparing the shifts with a reference tap and automatic operation is promptly locked and the lock is also released after retruning the taps in normal position.

CONSTITUTION: An order from a control section 13 is fed to a tap postion detection circuit 20 to hold a reference tap value. On the other hand, the order is transferred to tap switching transformers 12A, 12B at on-load to make taps rise and fall and the shifts of the taps are individually detected 21A, 22B to compare the shifts with a reference tap value. If the shifts are below the reference, auxiliary relays 18, 19 are released and if the shifts are over the reference, auxiliary relays 18', 19' are released. And individual tap shifts are detected 21A, 21B to detect 14 an overall shift and an auxiliary relay 16 is operated by a timing limiter 15 and normally closed contacts 26 are opened to lock automatic operation. Furthermore, a correction order for shifts is automatically and repeatedly fed to the transformers only by the predetermined means to promptly correct the shifts to a normal value to give an alarm when correction is impossible. This composition enables heightening of automatic operation efficiency.