To effectively utilize rainwater falling on a park, a schoolyard, or the like heretofore discharged wastefully and to reduce the occurrence of submersion and flood of land.

A part of rainwater collected in a final water storage tank 4 via respective primary storage layers 2a-2c is directly led into a purifying device 10 for purifying treatment via a primary water extraction pipe 6 or a secondary water extraction pipe 7 and a filter 9 and utilized as sprinkling and toilet water. Alternatively, the rainwater is once stored in a large-sized water storage tank 11, and the stored water is extracted and suitably purified in the purifying device 10 for use. Further, the rainwater is purified beforehand in the purifying device 10, and the purified water is stored in the water storage tank 11 of large capacity and utilized as water for living in case of a disaster or the like or utilized as potable water through an advanced sterilization treatment.