A combustion chamber 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 capable of resupplying the working gas into the combustion chamber by causing the working gas to circulate from an exhaust side to a suction side of the combustion chamber, supplying means that supplies the reactant, a concentration detection means capable of detecting a concentration of the reactant in the circulation path, and a supply control means that sets at least a first supply rate of the reactant by the supplying means in a time for starting based on the concentration of the reactant detected by the concentration detection means before a start of supplying of the reactant by the supplying means are included and so proper starting can be assured.