PURPOSE: To facilitate changing the layout and reduce the facility cost and running cost by disposing multiple blocks having a notch on floor slab, and providing a grating that forms an air duct between the floor slab and blocks.

CONSTITUTION: The floor 21 comprises a floor slab 22, multiple rectangular box-like blocks 23 which have an annular notch in the upper rim and are relocatable, and a grating 24, and an air duct is made up by the floor slab 22, blocks 23 and grating 24. Therefore, when the layout of one of the semiconductor manufacturing systems 5 is changed to a large size semiconductor manufacturing system, it can be completed in a short period of time by simply changing the arrangement of the blocks 23. Further, the construction of the floor 21 does not employ any special method, but employs the prefabricated blocks 23 etc., and so, the construction work is simplified and completed earlier. Since HEPA filters 30... are disposed away from a position directly above the air duct 27 in the wafer work area 4, the air volume to be air conditioned can be made small, resulting in the downsizing of the air conditioning system and the reduction of running cost.