An adapter with an insertion guide for connection between a porcelain tube and a branch tube is provided at an upper end portion of the branch tube. The adapter with the insertion guide includes an insertion guide portion extending in an axial direction of the branch tube. After inserting a lower end of a central conductor into an insertion portion of an arc-extinguishing chamber, a flange of the porcelain tube is inserted into the adapter with the insertion guide by guidance of the insertion guide portion. Accordingly, in a state where a bushing and the branch tube are self-aligned with each other, a lower end portion of the central conductor is inserted into the insertion portion.