WORKING GAS CIRCULATION ENGINE

A combustion chamber (CC) to which a reactant and a working gas whose specific heat ratio is higher than that of air are supplied and in which the working gas can expand following a reaction of the reactant, a circulation path (20) capable of re-supplying the working gas into the combustion chamber (CC) by causing the working gas to circulate from an exhaust side to a suction side of the combustion chamber (CC), supplying means (2) that supplies the reactant, a concentration detection means (3) capable of detecting a concentration of the reactant in the circulation path (20), and a supply control means (4) that sets at least a first supply rate of the reactant by the supplying means (2) in a time for starting based on the concentration of the reactant detected by the concentration detection means (3) before a start of supplying of the reactant by the supplying means (2) are included and so proper starting can be assured.