A method for treating a raw water containing a hardly decomposable substance, which comprises the following steps: a step (B) of adding an adsorbing agent to a raw water containing a hardly decomposable substance (a raw water to be treated) to allow said adsorbing agent to adsorb the hardly decomposable substance (an adsorption treatment step), a step (C) of separating a permeated liquid by the use of a filtration film, to concentrate the adsorbing agent having absorbed said hardly decomposable substance (a film filtration treatment step), and (D) a step of chemically decomposing the hardly decomposable substance being adsorbed in the concentrated adsorbing agent by a peroxide, without the operation for desorption from said adsorbing agent (a chemical decomposition step), and, by the above steps, concentrates hardly decomposable substances such as dioxins contained in a contaminated water (the raw water to be treated) and converts the hardly decomposable substances to harmless substances. The above method can be applied also to a raw water containing a reducing substance such as a hydrogensulfite for neutralizing free chlorine, the application thereof is not limited by the properties of hardly decomposable substances being contained, and also it allows conversion of hardly decomposable substances to harmless ones with good efficiency at a low cost. |