To enhance breakage resistance by manufacturing a casing made of plastic by injection molding.

An incandescent lamp bulb 1 is formed with plastic by injection molding. A joining component is also formed with plastic by injection molding. In order to make a base 2 or an inserting joining part conductive, a hardly heat conducting layer, a metal pin covered with ceramic layer for example, is inserted into a molding product so that the metal pin makes the conductive joining of the inside and the outside of the molding product possible, after the molding product has been taken out of a molding tool in injection molding. Accordingly, the bulb 1 and the base 2 are injection-molded integrally. As plastic, polycarbonate, polyamide, or polymethacrylate having a softening point higher than 150°C advantageous.