NOVELTY - Conversion of carbon dioxide to methane by using anaerobic microorganism in ballast tank of ship, comprises e.g. culturing anaerobic microorganism in ballast tank of ship, introducing carbon dioxide generated from the ship into the ballast tank, converting carbon dioxide into methane gas by using biological carbon dioxide-methane gas conversion process of anaerobic microorganism, and collecting the converted methane gas from the ship to supply as energy source.    USE - The method is useful for conversion of carbon dioxide to methane by using anaerobic microorganism in ballast tank of ship (claimed).    ADVANTAGE - The method reduces carbon dioxide by converting it into methane gas, which can be utilized as energy source.    DETAILED DESCRIPTION - Conversion of carbon dioxide to methane by using anaerobic microorganism in ballast tank of ship, comprises either: culturing anaerobic microorganism in ballast tank of ship, introducing carbon dioxide generated from the ship into the ballast tank, converting carbon dioxide into methane gas by using biological carbon dioxide-methane gas conversion process of anaerobic microorganism, and collecting the converted methane gas from the ship to supply as energy source; or processing an organic waste and simultaneously producing hydrogen as a reducing agent required during carbon dioxide-methane conversion by biologically degrading an organic waste in a hydrogen production unit (1), separating out hydrogen gas produced from the organic waste in a gas separation unit (2), supplying carbon dioxide into the ballast tank by collecting carbon dioxide generated in the ship using a carbon dioxide supply unit (3), dissolving carbon dioxide and hydrogen in a gas dissolution device (4), converting carbon dioxide into methane gas by using biological carbon dioxide-methane gas conversion process of anaerobic microorganism cultured in the ballast tank within a carbon dioxide-methane gas conversion device (5), and supplying converted methane gas as an energy source in a methane collection unit. An INDEPENDENT CLAIM is also included for a carbon dioxide to methane conversion system, comprising the hydrogen production apparatus, the gas separation apparatus, the carbon dioxide supplying unit, the gas dissolution device, the carbon dioxide-methane conversion device and the methane collection unit.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic process flow for conversion of carbon dioxide to methane by using anaerobic microorganism in ballast tank of ship (Drawing includes non-English language text).    Hydrogen production unit (1)    Gas separation unit (2)    Carbon dioxide supply unit (3)    Gas dissolution device (4)    Carbon dioxide-methane gas conversion device (5)