A method of treating hardly decomposable harmful substance comprising (A)
a step for adding coagulant into wastewater containing a hardly
decomposable harmful substance, (B) a step for performing solid-liquid
separation which separates a solid containing the hardly decomposable
harmful substance from a liquid, and (C) a step for decomposing the
hardly decomposable harmful substance by bringing peroxide into contact
with the separated solid. Efficiently and safely treating the hardly
decomposable harmful substances by means of low-cost facilities realizes.