PURPOSE: To remarkably simplify the dust dumping operation of each story in the construction process of a multistory building by providing opening areas passing through vertically, on the slabs of respective stories, and arranging a bucket conveyer in the opening areas.

CONSTITUTION: In the construction process of a multistory building 12, opening areas 16 passing through vertically... are formed on floor stabs 14... of respective floors at the stage of completing construction of the floor slabs 14... on each story. A bucket conveyor 16 then arranged between these opening areas 16 and a duct collecting unit 30 is provided on the bottom part of the bucket conveyor 18. Dust on plural upper stories is conveyed to the lower stories through the bucket conveyor 18. It is desirable to provide protection fences 34... around the fringes of the opening areas 16 in a manner of convering the bucket conveyor 18.