The invention is a water purification system through denitrifying reduction with faculative anaerobes in which a nitrate harmful in the breeding of creatures such as tropical fishes and coral for appreciation and fishes and shellfishes for food and the like is decomposed and denitrifying-reduced in a short time. In this case, an aerobic circulating pathway of water reduced through a filter using aerobes is separately arranged from an anaerobic circulating pathway of water reduced through a closed type anaerobic filter device (10) using anaerobes. A nitrate produced in the aerobic circulating pathway is exuded into the anaerobic circulating pathway through powdery sand (5). This nitrate is decomposed in the anaerobic circulating pathway. A nitrogen gas inside the anaerobic circulating pathway produced in the decomposition of the nitrate is discharged at an isolated state so as not to contact with water in the aerobic circulating pathway.
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