Recently, the research and the development of the technology to use energy effectively are done. However, a part of the steam generated with the boiler of the ship is thrown away without being used. Up to date, there was no effective solution that used such low-pressure steam. Therefore, the energy saving is expected in the ship in using the energy of low-pressure steam. In this report, the prototype steam engine that utilizes low-pressure steam is developed. In addition, this steam engine uses new cam that is called crown cam. Firstly, operating principle of prototype steam engine is described. Secondly, the design specification of prototype steam engine is described. Finally, structure of mechanism of prototype steam engine is shown.