Provided is a steam engine capable of obtaining motive force by using thermal energy. The steam engine, wherein a piston is slidably arranged inside a cylinder, a crank shaft is interlocked and coupled with the piston, and the rotating action of the crank shaft is linked with the reciprocal sliding of the piston, is characterized by: having arranged therein a cylinder capable of receiving heat from a high-temperature body
forming heat-conductive surface sections having a heat-exchange function between the cylinder and the piston reciprocally sliding inside the cylinder
heating these heat-conductive surface sections using the high-temperature body
spraying working fluid, being high-pressure, high-temperature superheated fluid, via an injector to inside a flash space sealed by the cylinder and the piston which is near top dead center
and, as a result, a certain ratio of superheated water becoming steam inside a space having a lower pressure than the saturation pressure of the superheated fluid, the steam expanding and causing the piston to operate, the non-vaporizedsuperheated fluid colliding, in a droplet state, with the inside surface of the heat-conductive surface sections forming the flash space and boiling and becoming steam, and the steam expanding and causing the piston to operate.