In a steam engine having a heating device, a cooling device, and an output device, the output device comprises a piston reciprocally moving by a self-excited fluid vibration of a working fluid in a fluid container. The piston is reciprocally moved by the output device for a certain period before starting an operation of the steam engine, so that the working fluid is moved to an inside space of the heating device. Since the working fluid is surely heated and vaporized by the heating device, the fluid vibration is stably started, and as a result, the operation of the steam engine can be smoothly started.