A working gas circulation engine (1, 201, 601, 701, 801, 901, 1001, 1101, 1201) comprising: a combustion chamber expanding a working gas that is Argon or Helium having a specific heat ratio higher than that of air due to a combustion between an oxidizing agent and a fuel; a circulation path that circulates the working gas from an exhaust side to a suction side of the combustion chamber and that is capable of supplying the working gas again to the combustion chamber; a pressure-reduction target section (2, 20, 3, 10b) that is a crank chamber that is a space other than the combustion chamber, a suction port (11 b) and an exhaust port communicating with the combustion chamber which are included in an engine body, and an internal pressure of the space possibly varies according to an operation state to become higher than a predetermined pressure; a feed apparatus (100, 600, 800, 1000) capable of feeding a gas in an inside of the pressure-reduction target section (2, 20, 3, 10b) whose pressure is to be reduced, to an outside; and a control means that operates the feed apparatus (100, 600, 800, 1000) to feed the gas in the inside of the pressure-reduction target section (2, 20, 3, 10b) to the outside, when a pressure in the inside of the pressure-reduction target section (2, 20, 3, 10b) is higher than a predetermined pressure set in advance, wherein when a pressure in the crank chamber detected by a crank-chamber internal pressure sensor is higher than a first predetermined pressure, the control means operates the feed apparatus (100, 600, 800, 1000) to feed the gas in the inside of the pressure-reduction target section (2, 20, 3, 10b) to the outside. a combustion chamber expanding a working gas that is Argon or Helium having a specific heat ratio higher than that of air due to a combustion between an oxidizing agent and a fuel; a circulation path that circulates the working gas from an exhaust side to a suction side of the combustion chamber and that is capable of supplying the working gas again to the combustion chamber; a pressure-reduction target section (2, 20, 3, 10b) that is a crank chamber that is a space other than the combustion chamber, a suction port (11 b) and an exhaust port communicating with the combustion chamber which are included in an engine body, and an internal pressure of the space possibly varies according to an operation state to become higher than a predetermined pressure; a feed apparatus (100, 600, 800, 1000) capable of feeding a gas in an inside of the pressure-reduction target section (2, 20, 3, 10b) whose pressure is to be reduced, to an outside; and a control means that operates the feed apparatus (100, 600, 800, 1000) to feed the gas in the inside of the pressure-reduction target section (2, 20, 3, 10b) to the outside, when a pressure in the inside of the pressure-reduction target section (2, 20, 3, 10b) is higher than a predetermined pressure set in advance, wherein when a pressure in the crank chamber detected by a crank-chamber internal pressure sensor is higher than a first predetermined pressure, the control means operates the feed apparatus (100, 600, 800, 1000) to feed the gas in the inside of the pressure-reduction target section (2, 20, 3, 10b) to the outside.