PURPOSE: To install a ceiling mount luminaire easily and safely without imposing extra load on a hook fitting by sandwiching the top board of the luminaire between a movement regulating member and a fastening nut portion.

CONSTITUTION: The hook fitting P1, P2 of an adaptor device 1 are respectively fitted to a ceiling body. A luminaire is surface mounted on a ceiling by screwing a tightening nut portion 3. Even when the adaptor device is fitted to a hook ceiling body with some thickness, and even if the tightening nut portion 3 is screwed, a top board is not allowed to warp. Because, a movement regulating member 4 is arranged on the lower face position of the hook ceiling body so that the position of the top board is regulated at the position of the move regulating member 4.