PURPOSE: To prevent water leakage for a long time by engaging rails each having an engaging portion which are provided at an optional interval on top of a floor body and the floor top faces between each of these rails with metal sheets for preventing water leakage and forming a surface layer with mortar, etc.

CONSTITUTION: A plurality of engaging rails 7 each having an engaging portion 6 are provided at an optional interval and fixed so as to plurally divide the top face of the floor 1 body while a plurality of metal sheets 8 cover the top face of each floor 1 body between each engaging rail 7 so as to prevent water leakage. An engaging portion 15 which is engaged with the engaging portion 6 of the engaging rail 7 is formed on a site toward one end portion of the metal sheet 8, a fixing means 9 closes and fixes the mutually adjacent end portions of the metal sheets 8, and a surface layer 10 is formed on top of the metal sheets 8 with mortar or concrete. Thereby, even when used for a long time, the metal sheets 8 are not deteriorated, surely preventing water leakage.