To provide sufficient draining performance, safely accessible from any position at the peripheral part of a floor pan since there is no height difference at the peripheral part of the entire floor pan, have no possibility that aged persons or handicapped persons miss their feet and fall, and eliminate a sense of incongruity caused by the conventional inclination of the entire floor pan by minimizing the height of the peripheral part of the floor pan.

Drain holes 2 are formed in a floor pan 1 with drain holes for forming a floor surface by laying. The drain holes 2 are formed at the center of a drain unit 11 where a predetermined area inducing to drain to one drain hole 2 is one drain unit 11. A drain hole cover 6 with an area 1/3 to 1/5 of the area of one drain unit 11 is put on the drain hole 2 so that water can be drained. A water-sealing means 5 is installed in each drain hole 2. A drain tube 22 or the like for guiding hot water to a drain passage is connected to each drain hole 2.