Working gas circulation engine system

The objective of the present invention is to provide an engine system that has superior thermal efficiency in an engine having a fuel used that is a fuel containing carbon, such as heavy oil, natural gas, or the like. This working gas circulation engine system (1A-1F), in which a working gas containing argon circulates within a system extending from the exhaust section of the engine to the gas supply section, is characterized by being provided with: the engine (10), which has a fuel containing carbon as the used fuel; a condenser (22) for eliminating moisture within the fuel gas discharged from the engine; an excess gas discharger (24) for discharging to outside the system a portion of the fuel gas discharged from the engine; a carbon dioxide elimination means (25, 27) for discharging to outside the system the carbon dioxide within the fuel gas discharged from the engine; and an oxygen supply means (26, 27) for supplying argon and oxygen contained in the air outside the system into the system.