PURPOSE: To decrease the number of working processes for an insulation test of a bushing, by attaching previously a metallic sheath to the insulating gas side of a wall-piercing bushing to be tested, enclosing the insulating gas into the sheath and set the sheath side into a test tank filled with an insulated fluid.

CONSTITUTION: A metallic sheath 4 containing the insulating gas 6b is attached to the insulating gas side of a wall-piercing bushing 2 to be tested for its insulation. Then this sheath side 4 is put into a test tank 11 filled with an insulated fluid 5b. For the connection between a test bushing 1 and the bushing 2, the shields 10a and 10b are attached at the parts of connection to prevent the corona occurrence. In the same way, a shield 10c is attached to the insulating gas side of the bushing 2. The bushing 2 can be set in and taken out along with the bushing 1, and the work to exhaust a fluid 5b within the tank 11 can be excluded. Furthermore the tank 11 can be installed under the ground. In such way, the working efficiency is increased for an insulation tester of bushing.