PURPOSE: To promote an improvement of efficiency for a work in operational field, by installing to fit a lighting duct in a slit with a flange of the duct made touching to a ceiling leaf of unit length made with the slit formed in surfaces of broad band width.

CONSTITUTION: A decorating ceiling leaf 10 of unit length and width is constructed with a longitudinally long slit 11 formed in center surfaces of wide band width. A lighting duct 12 of unit length is constructed with conductors 15, 15 provided to be housed in a duct D of square-shape, which is constructed with flanges 13, 13 protrusively provided at the both sides and a plug insertion port 14 formed in the lower part. The lighting duct 12 will be assembled to be mated to the ceiling leaf 10, with the flanges 13, 13 made touching surfaces of wide band width and installed into the slit. The lighting duct 12 will be secured to a mounting plate 16, installed to the ceiling leaf 10, by screws being screwed into the both. And for the lighting duct, with a lead wired connecting unit 17 installed to its one end, is provided a connection of power source