Abstract: The present invention is directed to provide a water treatment technology able to perform advanced purification treatment by efficiently removing synthetic organic compounds such as surface activators for use in cleaners and agricultural chemicals present in polluted water having a relatively low load of pollutants such as BOD. In order to accomplish this objective, water to be treated is brought in contact with a assimilable filter medium under aerobic conditions, said assimilable filter medium being composed by adhering a material to be assimilated that has specific assimilability for the target microorganisms to a porous base material such as charcoal. As a result of using this type of assimilable filter medium, specific microorganisms which are highly efficient for removing the synthetic organic pollutants to be removed can be utilized selectively, thereby enabling efficient removal of synthetic organic compounds.