Ribs formed on two sides of a case are press-fitted into grooves formed in the holding faces of a holding member. Two case bodies constituting the case have rib divisions formed by dividing ribs in their longitudinal directions. Accordingly, the two case bodies are brought into contact with each other in a wide range on the divided surfaces of the rib divisions, the divided surfaces extending in the longitudinal directions. When the ribs are press-fitted into the grooves, the divided surfaces in contact with each other are brought into pressure-contact with each other in a wide range.