PURPOSE: To easily open or close a cover placed on a base by fingers and to roughly align the surface of the upper wall of the cover with the upper wall surface in the middle of the base when a fastener and mounting hole on either mounting end of the base are covered with the cover.

CONSTITUTION: Engaging side walls 16 each having a hook-shaped engaging part 17 are provided on the opposite mounting ends of a base, respectively, while an elastic engaging holder 24 having fork-shaped cross section is formed in each cover, and the holder 24 is compressively engaged against the part 17 to rotatably mount the cover to the base. For mounting the cover, the surface of an end 21a of an upper wall 21 of the cover is brought into contact with the bottom upper wall 14 of each of the opposite mounting ends in such a manner that the surface of the end 21a is roughly aligned with the of an upper wall 15 in the middle of the base, and the cover has vertical side walls 23 adapted to be placed adjacent to the upper wall 15.