PURPOSE: To facilitate the test by a method wherein among manholes which are prepared on a tank for connection between an oil-gas bushing and a lead wire immersed in oil two of them belonging extremes of phases are built facing outward refering central one.

CONSTITUTION: Among manholes 8 which are prepared on a tank 3 for connection between an oil-gas bushing 4 and a lead wire immersed in oil both manholes belonging extremes of phases are prepared facing outward having an angle refering to the center line of the tank. On testing a testing pocket 9 and an oil-air bushing 10 are attached individually on each manhole 8 and a lead wire 11 is connected individually between a corresponding bushing 10 and the oil-gas bushing 4. On the other hand a temporary tank 12 is applied on the gas filled side of the oil-gas bushing and it is filled with insulating gas. In this arragenment with a testing voltage applied on an air side terminal of the bushing 10 individual test of a transformer is attained.