An adhesive layer, an insulating layer and a copper foil are laminated together on both surfaces of a metallic base material by way of for example thermal press molding. In this case, openings (window holes) are formed in opposed positions on a portion of the adhesive layer. A circuit pattern is formed by etching on the copper foil in this state, followed by an external shape machining step of executing separation treatment reaching the metallic base material in predetermined positions including the openings. After that, a part of the insulating layer is cut off along the edge of the opening to obtain a circuit board with the end of the metallic base material exposed.