In a circulation system of treatment-object water (waste water), a water treating apparatus and a pH adjuster are provided. A carbon fiber that can collect at least microorganisms is disposed in the water treating apparatus. The carbon fiber is immersed in the treatment-object water, and potential is applied to the carbon fiber. Further, pH of the treatment-object water is adjusted by the pH adjuster in a direction in which adsorption of microorganisms in the treatment-object water to the carbon fiber is facilitated. As a result, the microorganisms are strongly attracted and adsorbed to the carbon fiber.