A steam engine of a simple constitution capable of efficiently obtaining mechanical energy not only from a heat source of a high temperature but also from various heat sources in a low-temperature state such as the exhaust heat of an internal combustion engine. The engine comprises a displacement engine 1 having a rotor 13 that rotates, a steam-generating portion 2 arranged thereunder and a condenser 3 arranged thereover. Water heated in the steam-generating portion 2 becomes the steam which is fed into the displacement engine 1, enters into an operation chamber that moves up, rotates the rotor 13 and is, thereafter, fed into the condenser 3 where it is condensed to turn into the condensate. The condensate falls on the rotor 13 on the lower side due to gravity, and is refluxed into the steam-generating portion 2 through the operation chamber that moves downward with rotation of the rotor 13. The steam engine efficiently operates even at low speeds, requires no pump for refluxing the condensate, and is constituted in a compact size.