PURPOSE: To effectively separate and remove the water-soluble org. matter, etc., from breeding water by alternately arranging a partition having an opening at its upper part and a partition having an opening at its lower part in a vessel to form a bubble passage and providing a water supply means and a bubble forming means at the starting point of the bubble passage and bubble separating and discharging means at the terminal point.

CONSTITUTION: The partition 2a having an opening at its upper part and the partition 2b having an opening at its lower part are alternately arranged in the vessel 1 to form the bubble passage 3. The water for a breeding water tank 4, etc., is supplied from a water supply pipe 5 to the starting point 3a of the passage 3, and air is injected from an air stone 7 into the water. Consequently, innumerable fine bubbles are formed, and modified by the water-soluble org. matter and fine granular material in the water to form innumerable durable bubbles having high surface tension which move upward, and the bubbles are maintained even after emerging from the water. The bubbles are gradually coalesced in the passage 3 to form large bubbles and reach a bubble discharge port 8. Consequently, the watersoluble org. matter and fine granular material are effectively separate and removed by the bubbles.