PURPOSE: To facilitate maintenance and inspection, and to enhance safety of a gas insulated on-load tap changing transformer by a method wherein a tap selector is inserted in oil in an oil chamber provided by partitioning the inside of a tank, and is connected to the main body with connecting wires crossing over a partition between the main body.

CONSTITUTION: The partition 14 opening the upper part thereof is provided in space at one side in the tank 11 accommodating the transformer main body 1 filled up with insulating gas to form the oil chamber 13, and insulating oil 15 is poured therein. An insulating cylinder 18 accommodating a diverter switch 19, a breaking part 20 thereof, and an energy-accumulating driving device 21 is hung in space on the oil face, and the tap selector 22 accommodated in oil is equipped at the lower end part of the insulating cylinder 18. Connection between the tap selector 22 and the main body tap winding 2 is attained by the connecting wires 23 arranged by crossing over the upper end of the partition 14. Accordingly, because inspection and filling up of grease of the switching 19. the breaking part 20, etc., accommodated in the insulating cylinder 18 can be attained by opening a cover 17 for inspection and drawing them up to the upper part, no fear to generate trouble exists, and safety thereof can be enhanced.