PURPOSE: To secure the safety in case of fire by interposing an isolating member isolating a space down below a skylight between each beam member supporting the skylight and the skylight in such a way that respective sheet type plates of the isolating member can be spread.

CONSTITUTION: Each guide frame 13 for the sheet type plates 41, 42, 43...4n of an isolating member 4 to be spread while being slid, is interposed between each beam member 12 supporting the whole ridge 11 and each lower end of the ridges 11 every ridge 11 of a skylight 1, and a housing box 5 for the isolating member 4 is provided for each end side of the ridges 11. The isolating member 4 is composed of plural sheet type plates 41, 42, 43...4n each of which is made of steel applied by insulation, and these sheet type plates are normally housed in the housing box 5 while being over-lapped. On the other hand, heat sensors 31 and IR ray sensors 32 for fire detection are provided for the wall surface around the stair well every floor of the building itself 3. And the isolating member 4 permits its sheet type plates to be automatically spread according to the detected signals from these heat sensors 31 and IR ray sensors 32.