In an engine 10, oxygen, hydrogen serving as fuel, and argon gas serving as a working gas are supplied to a combustion chamber 21. An upstream condenser section 70 condenses water vapor contained in an exhaust gas to yield primary condensed water, through heat exchange of the exhaust gas from the combustion chamber with the ambient air, and discharges, as a primary-condensed-water-separated gas, a gas obtained by separating the primary condensed water from the exhaust gas. The primary condensed water is stored in a water storage tank 100. A downstream condenser section 80 further condenses water vapor contained in the primary-condensed-water-separated gas to yield secondary condensed water, through utilization of latent heat of vaporization of condensed water stored in the storage tank 100, and discharges a gas obtained by separating the secondary condensed water from the primary-condensed-water-separated gas. This avoids a significant drop in ratio of specific heats of the working gas.