PURPOSE: To increase the content of oxygen of a natural water area and thereby purify the water by allowing a water current from a natural water area to circulate within a facility for sport purposes, then pumping the water to an area above a tower structure and releasing the water to the natural water area after permitting it to fall.

CONSTITUTION: A leisure facility B based on a water stream is built near a natural water area A under environments where a water stream is restricted like a dammed lake or a swamp, or where a water stream is blocked. The water stream conducted from the natural water area A is caused to circulate in the facility B for sport purposes, and part of or the whole water stream is pumped up to an area above a tower structure 5. After that, an aeration phenomenon is generated in the natural water area A by allowing the pumped up water to cascade in the natural water area A. Subsequently, the air is dissolved into the water to increase the content of oxygen in the water of the natural water area A. That is, an extremely satisfactory water purification action takes place.