A cover is coupled to a base with the inner surface of the cover opposed to the inner surface of the base. In this case, a rib standing from the inner surface of the cover is guided along the contour of a printed wiring board fixed to the inner surface of the base. The method allows the printed wiring board on the base to guide the movement of the rib of the cover. The printed wiring board is utilized to align the cover with the base with a higher accuracy. The cover is thus reliably prevented from colliding against an electronic component mounted on the printed wiring board. This results in a reliable avoidance of damage to the electronic component.