To form a recessed part without leaving the mark of a seal on the surface of a floor.

This floor is provided with a doughnut-shaped expansion ring 11 to be expanded by injecting air and to be shrunk by discharging air, a middle plate part 15 composed of two portions which are fixed on both the sides of the expansion ring 11 at peripheral edge parts so as to close a doughnut-shaped space, while having a stuck portion 19 on the center, a floor plate part 20 fixed on one side of the expansion ring 11 at a peripheral edge part so as to cover the outside of the middle plate part 15 and a sealed space 21 formed between the middle plate part 15 and the floor plate part 20, and when the expansion ring 11 is expanded, the center of the floor plate part 20 is recessed toward the central stuck portion 19 of the middle plate part 15.