A method for treating a hardly-decomposable-substance-containing water which including: (B) adding an adsorbent to water containing a hardly decomposable substance to cause the hardly decomposable substance to be adsorbed on the adsorbent (adsorption treatment step); (C) separating a permeated liquid through a filter membrane to concentrate the adsorbent which has adsorbed the hardly decomposable substance (membrane filtering treatment step); (D) decomposing the hardly decomposable substance which has been adsorbed on the concentrated adsorbent (hardly decomposable substance decomposition step); and (E) returning the adsorbent after the decomposition of the hardly decomposable substance to the adsorption treatment step (B) (adsorbent returning step); and an apparatus for the treatment .