PURPOSE: To provide a reciprocating hydrogen-fuel engine of such constitution as to improve cycle heat efficiency by the use of high-temperature operating steam in a system utilizing a hydrogen-fuel engine and to sufficiently cope with average temperature around the combustion chamber of the engine by the use of constituent members made of ordinary metallic material.

CONSTITUTION: This hydrogen-fuel engine is of a compound system of decelerating-synthesizing the output of a hydrogen/oxygen combustion reciprocating engine 3 or a reciprocating engine and the output of a turbine driven by the exhaust of the engine, and the hydrogen/oxygen combustion reciprocating engine 3 is formed being provided with three types of injection valves, that is, a liquid hydrogen injection valve 31, a liquid oxygen injection valve 32 and a water injection valve 33, a spark plug 34 and an exhaust valve 35 disposed at a cylinder head. This compound system is a closed cycle hydrogen-fuel engine system formed by connecting the hydrogen-fuel engine 3, a heat exchanger 5 and a condenser 6 by a closed circuit.