To adjust a water level without interrupting the switches of a water receiving pump and a water sending pump and wasting water resources by attaching a returning branch pipe to an open system water tank and providing a value which adjusts the volume of returning water according to the water level in the open system water tank.

The open system water tank 1 is arranged between a pool 14 and a solid-liquid separating device 15 and connected by water sending and receiving pipes so that water returns from the pool 14 to the solid-liquid separating device 15, open system water tank 1, and then pool 14. The water sending pipe 4 connecting the open system water tank 1 and pool 14 is provided with the branch pipe 4 allowing part of water sent from the open system water tank 1 to the pool 14 to reflow to the open system water tank 1. On the upstream side of the water sending pipe 4, the water sending pump 3, a butterfly valve 7, and a check valve 13 are fitted in order and on the upstream side of the branch pipe 5, a ball cock valve 8 and a ball tap valve 6 are fitted in order. Further, the open system water tank 1 is provided with a liquid gauge 9, an overflow pipe 10, a drain pipe 11, and a drain valve 12.