PURPOSE: To keep the cleanlines of the water of a pond or lake in an extremely high state by providing a water treatment means for sewage or various waste waters, a water channel connecting the pond or lake and a means taking in the water of the pond or lake to modify the same.

CONSTITUTION: A water treatment means having a water purifying tank 9 receiving sewage or various waste waters from an inflow line 13 to purify the same is provided and a water modifying device taking in the water from a pond or lake 10 to modify the same is provided. Water is taken in from the intake part 2 of the pond or lake 10 to be supplied to a water receiving tank 4 through the passage of a water feed pipe 3 and supplied to the water modifying device arranged inside or outside the water receiving tank 4 to be modified. The modified water is circulated through the water feed pipe 3 or other water feed pipe until the water of the pond or lake 10 keeps desired cleanliness to improve the quality of the water of the pond or lake 10.