This study investigates the characteristics of water purification and the denitrification of PVA-immobilized photosynthetic bacteria by focusing on the long-term purification of fish breeding ponds and the maintenance of the denitrifying ability of the photosynthetic bacterium, Rhodobacter sphaeroides. In addition, the accumulation of nitrite in the water by PVA-immobilized photosynthetic bacteria was also analyzed in order to elucidate the denitrifying activities in PVA gel beads.