In integrally forming a positioning convexity on an inner surface of a case, breakage during a fall and the like is prevented by increasing the strength of the positioning convexity without causing a defective appearance during the molding of the case. In performing the positioning of a top cover and a built-in frame by fitting together a positioning concavity formed in the built-in frame and a positioning convexity which is integrally formed on an inner surface of the top cover, the positioning convexity is formed from a rib in the shape of a box or the like. To lessen stress concentration, the positioning convexity is disposed in the vicinity of a screw fixing section. To improve assemblability, a curved surface or an inclined surface which guides the fitting into the positioning concavity is formed at a leading end of the rib which constitutes the positioning convexity.