PURPOSE: To prevent the degradation of insulation in insulated support material by providing a sealed ring of the earth potential surrounding the support material on the outside of the insulated support material.

CONSTITUTION: The outside of the insulated support material 5 is provided with the sealed ring 11 of the earth potential surrounding the support material. In such an insulated device, even if a voltage is applied to breaking portion, no equipotential line exists in its neighboring area because the sealed ring 11 is provided. Thus, the electric field of the outside circumference F of insulated support material is weakened and therefore the movement of the conductive impurities 12 can be avoided, whereby insulation degradation along the support material can be prevented and also the electric field of the upper part G of the buried metal 6 can be weakened, suppressing the occurrence of trigger.