To insolubilize and remove eutrophication components by filling a treating material formed by mixing an insolubilizing material which forms an insoluble body to react with the eutrophication components in water, with a carrier in a treating tank.

A 2nd tank 2 connecting to a 1st tank 1 provided with an aeration pipe 1p is formed from a preliminary zone 2a and a reaction zone 2b and a filter medium for a biological membrane is filled in the preliminary zone 2a. A treating material Pa is filled in the reaction zone 2b. As the treating material Pa, a gel type one making a gel as a carrier is used. The gel type treating material Pa has a net like structure having many through holes and carries a material insolubilizing the eutrophication components such as phosphorus in the water to be purified, for example, iron and magnesium. A 3rd tank 3 and a 4th tank 4 filled with the filter medium Fb using charcoal are connected to the downstream of the 2nd tank 2. As the result, phosphorus is effectively removed and when stirring or the like is used jointly, the removing efficiency is further improved.