WORKING GAS CIRCULATION ENGINE

A working gas circulation engine includes a combustion chamber (10a) that can expand a working gas, which has a specific heat ratio higher than that of the air, with the combustion between an oxidizing agent and a fuel; a circulation path (20) that can allow the working gas to circulate from a suction side to an exhaust side of the combustion chamber (10a) and can supply the same again to the combustion chamber (10a); a feed means (100) that can feed the gas in a pressure-reduction target section (2), which is the section whose pressure is to be reduced, to the outside, and a control means (53) that operates the feed means (100) for feeding the gas in the pressure-reduction target section (2), when the pressure in the pressure-reduction target section (2) is higher than a predetermined pressure that is set in advance.