To allow performing insulation dielectric strength test regarding a neutral point, without recovering insulation medium in a tank.

An insulation spacer 30 is fixed in a neutral point pocket 22 of a tank 10, and with an opening 20 closed with the insulation spacer 30, a test insulation bushing 42 is detachably fixed to a flange 28 if the neutral point pocket 22. Both ends of a test connection lead 38 are connected to the insulation spacer 30 and a lead in the test insulation bushing 42, respectively, and the electrode exposed above an end surface of an insulator 48 is applied with a voltage for insulation dielectric strength test for insulation dielectric strength test regarding the neutral point.