A steam engine capable of efficiently obtaining mechanical energy from not only heat sources of high temperature but also various heat sources in a low temperature state such as exhaust heat from internal combustion engines and solar heat by a simple structure. In the engine, a rotor (5) having a bent jetting tube (51) is rotatably supported in a closed container (1) filled with a liquid. A heating part (9) is inserted into a center cylinder (50) at the center of the rotor, and a high temperature fluid is passed therethrough to evaporate the liquid sucked from the suction tube (52) of the rotor (5). The mixture of vapor with the liquid is jetted from the jetting tube (51) by the pressure of the evaporated vapor to rotate the rotor (5). A jetting check valve (53) and a sucking check valve (54) are disposed at the tip parts of the jetting tube (51) and the suction tube (52), respectively. The jetted vapor is led to a condenser (2) installed above the closed container (1), condensed, andrecirculated to the closed container (1). A vacuum pump (23) is connected to the condenser (2) to hold a pressure in the closed container at a saturated vapor pressure.