PURPOSE: To replace insulation resistance at a lead support section which is determined based on creeping pressure resistance of a support insulator whose pressure resistance is lower than liquid insulation with lead insulation not affected by lower creeping pressure resistance.

CONSTITUTION: An insulation arm 4a is installed to an opposite side of an adjacent grounding metal 3 for a lead core 1. The lead 1 and the grounding metal 3 are held with support members 4b and 4c by way of this insulation arm 4a. This construction forces an electric field orientation in a creeping direction of the support member 4c to be turned over on its way just as 12a and 12b are and provides insulation structure capable of preventing the development of creeping direction.