An image forming apparatus includes a shield case in which a circuit board mounted with an electric circuit is contained. The shield case has an electrical conductive case main body and an electrical conductive lid member. The case main body has an aperture formed in a part of one face. The lid member has one end turnably supported by the case main body and the other end fastened to the case main body by a screw when the aperture is closed. The case main body and lid member have an engaged part and an engaging part engaged with each other when the aperture is closed. The engaging part is formed so as to clamp the engaged part. The shield case is configured so that contact force between the engaging part and engaged part is increased, if the screw is rotated in a fastening direction when the aperture is closed.