A molded resin casing of an electronic part equipped with a flexible flat cable and internally accommodating a board on which are formed electric conductor patterns slidingly contacted by contacts of a slider of the electronic part, includes a flexible board having various electric conductor patterns formed on a synthetic resin film used as the board. The flexible board is formed integrally with a flexible flat cable comprising a synthetic resin film on which are formed electric conductor patterns electrically connected to the aforementioned various electric conductor patterns. When the molded resin casing is molded by injecting a molten synthetic resin, the flexible board is inserted in such a manner that the electric conductor patterns are exposed within the casing, thereby integrating the flexible board and synthetic resin casing, with the flexible flat cable extending outwardly from a side portion of the synthetic resin casing.