A mirror box formed of metal serves as a metallic portion constituting together with a plastic portion an essential part for imparting strength characteristics to a camera body. The metallic portion is provided with a base for a lens mount and with similar parts for assembling other element. During molding, some of the metallic segments of the lens mount base are inserted as a framework into the plastic portion so that the metallic and plastic portions are constructed in the form of a unit. This unit is then assembled with other portions as the base unit of the essential part to complete a single lens reflex camera body.