Multiple high-voltage side and low-voltage side electric conductors are formed from one sheet of a conductive plate in such a way that the multiple electric conductors are arranged in parallel to one another across an initial gap between the high-voltage side and the low-voltage side electric conductors. The multiple electric conductors are connected to one another via connecting portions. An intermediate portion of the connecting portion is deformed so as to reduce the initial gap to a smaller adjusted gap. Portions of the electric conductors as well as switching devices mounted to the electric conductors are sealed by sealing material. The connecting portions are cut away so that the electric conductors are finally separated from one another.