PROBLEM TO BE SOLVED: To provide a filtration system of a closed system water area in which production cost can be suppressed by simplifying a structure.SOLUTION: A filtration system of a closed system water area comprises: a nitrification tank 5 for nitrifying ammonia included in water in a closed system water area 2 a denitrification circulation tank 13 for denitrifying nitric acid included in the water in which the ammonia is nitrified in the nitrification tank a main pump 10 for circulating the water in the closed system water area through the nitrification tank a supply pump 10 for denitrification to supply a portion of the water in the nitrification tank to the denitrification circulation tank and an overflow pipe 24 for returning the water in the denitrification circulation tank to the nitrification tank. The denitrification circulation tank comprises a denitrification area 15 and a denitrification circulation area 16 partitioned by an inside partition 14, and a denitrification circulation path 22 and a circulation pump 21 for circulating the water in the denitrification circulation area to the denitrification area. The position lower than the upper end of the inside partition is set as a setting water level, and an overflow pipe has a base end positioned at the same level as the setting water-level.
