PURPOSE: To simplify a replacement work of a sample bushing, to shorten a gas collecting time, and to elevate the test efficiency, by installing a gas tank to flange metallic fittings on a gas insulating medium side of a bushing in advance, immersing the whole body is an oil tank, and executing its test.

CONSTITUTION: To a gas side of flange metallic fittings 5 of a sample bushing 1, a gas tank 13 is installed in advance, in a state that a gas insulating medium 12 has been sealed in its inside. Subsequently, a testing bushing 14 used for oil is prepared, and to flange metallic fittings 15, a fitting cover 4 of an oil tank 3 is installed. This testing bushing 14 is connected with the sample bushing 1 in one body. This unified bushing is immersed in the oil tank 3 filled with insulation oil 2, and the fitting cover 4 is installed to the oil tank 3. When a test is executed in this way, a gas connecting box becomes unnecessary, and the working time can be shortened remarkably.