PURPOSE: To prevent the falling-down of workers by a method in which a duct is set by piercing it through the floor of building and a falling preventive bar is provided in the inside of the duct.

CONSTITUTION: A sleeve 13 is attached through a fitting metal 12 to a concrete floor 11, and a heat insulator 14 is packed into desired places between the floor 11 and the sleeve 13. A cover 15 is provided on the upper end opening of the sleeve 13, and four falling preventive bars 16 formed by assembling two bars vertically and horizontally are set on the inside of the sleeve 13. The falling-down of workers to the inside of the sleeve 13 can thus be prevented.