A Fenton reaction catalyst including a reaction product as an active component, in which the reaction product is obtained by using roasted and ground coffee beans (in particular, coffee grounds) or tea leaves (in particular, tea dregs) as feedstocks for supplying a component having a reduction effect, and mixing the feedstocks for supplying the component having a reduction effect with an iron-supplying source containing divalent or trivalent iron in the presence of water. The Fenton reaction catalyst can be used to generate hydroxy radicals from hydrogen peroxide. The Fenton reaction catalyst can maintain iron in a divalent state for a long period of time.
