A molded resin casing of an electronic part equipped with a 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. A flexible board having various electric conductor patterns formed on a synthetic resin film is used as the board, the flexible board is formed integral with a flat cable including a synthetic resin film on which are formed electric conductor patterns electrically connected to the aforementioned various electric conductor patterns, and 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 flat cable extending outwardly from a side portion of the synthetic resin casing.