PROBLEM TO BE SOLVED: To provide a COdecreasing system by recovery/isolation of sea plankton which recovers and isolates plant plankton absorbing COto surely decrease the CO.SOLUTION: The COdecreasing system by recovery/isolation of sea plankton includes recovering plant plankton 1 with ciliary filtering member 4 separately filtering the plant plankton 1 caught together with seawater 2, and isolating the CO-absorbing plant plankton 1 under the ground to recover/isolate the CO.