PURPOSE: To prevent a trouble such as failures or electric leakage of apparatus provided on a basement, which is caused by water-leakage trouble in a working room, in a structure wherein clean air is allowed to circulate through the basement provided under the working room in a clean room.

CONSTITUTION: A basement 2 is provided at the lower part of a working room in a clean room, a floor 4 on a working area is formed of a metallic plate having a number of through-holes, and clean air is given as a laminar air downflow easy to control and is allowed to circulate through the basement 2. The floor on the basement 2 is divided in block form into a number of island zones 7 by a plurality of drainage ditches 6, and auxiliary apparatus not always necessary for the working room are arranged on each of these zones 7. Drainage basins 8 communicating with the drainage ditches 6 and drainage pipe-lines 9 are provided. In this way, since water falling on one of the zones does not spread to others of the zones 7 even if a water-leakage trouble occurs in the working room by any chance, damage extending over a wide range can be prevented.