A rotary 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 has a rotor 1 having a plurality of displacement chambers 11 provided in a sealed container 2 which is filled with a liquid. A steam-generating portion 4 is arranged under the rotor 1 and where the liquid vaporizes being heated by the exhaust heat of an internal combustion engine. The vaporized stem is jetted from a flow-out passage 42 toward the displacement chambers 11 of the rotor 1. The steam stays in the displacement chambers 11 and, therefore, buoyancy acts onto the displacement chambers 11 on one side of the rotor 1. The rotor 1 rotates to produce the rotational energy. The steam in the displacement chambers 11 is released in the sealed container 2 accompanying the rotation of the rotor 1, and is introduced into a condenser 3 where the steam is condensed and refluxes into the sealed container 2. The pressure in the sealed container 2 is maintained to be a saturated steam pressure by a vacuum pump 34. Therefore, the steam is formed despite the liquid has a low temperature to rotate the rotor 1.