To perform an easy moisture condensation-proofing work at low cost by sticking a foamed soft polyvinyl chloride sheet or hydrophobic polyurethane foamed sheet to the bottom surface of an urinal, and closely adhering it according to the irregular form of a trap bottom part.

A foamed soft polyvinyl chloride sheet 4 or hydrophobic polyurethane foamed sheet for covering the bottom part of a trap T is preliminarily stuck to the bottom part of the trap T prior to the setting of an urinal 1. The sheet is closely adhered according to the irregular form of the trap T bottom part, and the urinal 1 is set on a floor surface F in this state to interpose the stuck foamed soft polyvinyl chloride sheet 4 between the floor surface F and the trap T, whereby the moisture condensation on the trap T bottom part is prevented. Thus, dew condensation can be prevented to prevent the sweating on the floor surface F side.