To improve a sense of quality and reliability only by fitting an upper cover from an upper side of a lower cover to make a packing appropriately compressed at all their fitting parts for obtaining waterproofness.

An opening 1d of a lower cover 1 and a side wall 2b of an upper cover 2 are formed into trapezoidal shapes with upper widths (a), (e) wider than lower widths (b), (f), and only by fitting the upper cover 2 from an upper side of a lower cover, a packing 3 interposed between fitting parts 2d, 1e at left and right slopes on the side wall 2b and the opening 1d is appropriately compressed. If gradient angles of the fitting parts 2d, 1e on the left and right slopes are formed to be symmetric, the packing 3 can be compressed uniformly in right and left, thereby obtaining more improved waterproofness.