To provide an actuating gas circulating type hydrogen engine which can prevent a drop in a specific heat ratio of circulating gas (the gas which functions as actuating gas) supplied to a combustion chamber, and can be operated at high thermal efficiency even when atmospheric temperature is high and the amount of water vapor contained in the circulating gas, which has passed through a condenser, is large.

The engine 10 supplies oxygen, hydrogen as fuel, and the gas which consists of argon as the actuating gas to the combustion chamber 21. The condenser 80 condenses water vapor contained in the exhaust gas, and makes it condensed water. Then it discharges the gas, from which the condensed water has been separated, from an outlet 80b. Then the gas discharged from the outlet 80b passes through a first moisture absorber 90. In this case, the moisture contained in the gas is eliminated by a moisture absorption material which is stored in the first moisture absorber 90. The gas which has passed through the first moisture absorber 90 is supplied to the combustion chamber again.