Evaporator temperature control means (U) that, in order to make the temperature of steam generated by heating water using exhaust gas of an engine coincide with a target temperature, controls the amount of water supplied to the evaporator based on the flow rate of the exhaust gas, the temperature of the exhaust gas, the temperature of the water, and the temperature of the steam. Calculating an exhaust gas flow rate (Gmix) based on a fuel injection quantity, an air/fuel ratio, and a rotational speed of the engine improves the precision and the responsiveness of the calculation. Controlling the amount of water supplied to the evaporator based on this exhaust gas flow rate (Gmix) therefore improves the accuracy with which the steam temperature is controlled so as to make it coincide with the target temperature.