PURPOSE: To improve the efficiency of operation by stretching a ceiling panel over the upper section of a wall panel, bearing the ceiling panel from a lower section by a bearing member and conducting the operation inside the ceiling plenum.

CONSTITUTION: A wall panel 2 is disposed in upright on a wall-panel placing section 6a in the side edge section of a waterproof pan 6, and a ceiling panel 1 is stretched over the upper end of the wall panel 2, thus constructing a unit room. A bearing member 7 is brought into the unit room and the ceiling panel 1 is borne from a lower section, and an operator enters into the ceiling plenum 5 by a trestle 4, etc., and conducts ceiling-inside operation. Accordingly, the operator rides on the ceiling panel and can perform operation efficiently in an easy attitude.