In a steam motor with a piston engine, the piston engine is included in a closed steam circuit. This steam circuit includes a steam generator, a steam injector for injecting steam into the piston engine, a condenser for condensing the steam emerging from the piston engine to condensed water, and a water feeding pump for feeding the condensed water to the steam generator. The steam generator is heated by hot combustion gas from a combustion unit. The combustion unit burns fuel. The fuel is mixed with fresh air supplied by an air feeding device through a fresh air passage. The fresh air passage usually contains a first heat exchanger for pre-heating the fresh air by heat from the expanded steam emerging from the piston engine, and a second heat exchanger for pre-heating the fresh air by heat from hot waste gas emerging from the steam generator. In order to provide a particularly compact steam motor without adversely affecting the efficiency, a rotary piston engine is used as piston engine.