Abstract: A system for making use of rainwater falling on houses and buildings, which comprises a means for removing a predetermined quantity of initial precipitation from the rainwater collected form the roof surface of the building; and a purifier receiving the subsequent rainwater from which the initial precipitation has been removed. The purifier includes a filtering tank, a sterilization tank, and a supply tank connected to the sterilization tank. The filtering tank has a physical filter and a pH adjusting agent, the sterilization tank contains a primary reactive catalyst for producing active oxygen species, and the supply tank supplies aqueous hydrogen peroxide to the primary reactive catalyst. The system also has a storage tank that stores the purified rainwater supplied from the purifier. The storage tank contains a secondary reactive catalyst for decomposing and eliminating residual active oxygen species remaining in the purified rainwater supplied from the purifier.