To provide a method for installing and constructing a movable floor for an existing pool by which a movable floor device can be internally installed easily while maintaining the water depth of the existing pool and without breaking down the existing pool, and to provide the movable floor device used for the construction method.

A pool tank of the movable floor device has an opening part which is given to a shape similar to a plane shape of an opening part of the existing pool with a fixed floor, but somewhat smaller than that of the existing pool. The pool tank is fitted into the opening part of the existing pool. Then a cover is fixed to occlude a gap between the pool tank of the movable floor device having the substantially same height as the height from the fixed floor of the existing pool to the upper end of the opening of the pool, and the inner wall of the existing pool.