The working gas circulation engine includes a circulation route capable of circulating gas containing the working gas from an exhaust side to an intake side of a combustion chamber and resupplying to the combustion chamber and provided with a removing device to remove a product generated with a reaction from the circulating gas, a supplying device capable of supplying plural kinds of reactant gas to the combustion chamber or the circulation route, a pressure detecting device capable of detecting pressure in the circulation route, and a control unit that controls supply amount of at least one kind of the reactant gas to be supplied from the supplying device based on the pressure in the circulation route detected by the pressure detecting device, and performs pressure control to adjust the pressure in the circulation route.