When a temperature setting means (M1) sets the steam temperature at the outlet of an evaporator (10) on the basis of the temperature and flow rate of exhaust gas at the inlet of the evaporator (10), a temperature control means (M2) controls the steam temperature at the outlet of the evaporator (10) so that it is equal to the set temperature. Further, when a pressure setting means (M3) sets the steam pressure at the inlet of an expander (11) on the basis of the set temperature, a pressure control means (M4) controls the steam pressure at the inlet of the expander (11) so that it is equal to the set pressure. This ensures that the steam at a temperature at which the total efficiency of the evaporator (10) and expander (11) is at maximum is produced in the evaporator (10) and that the steam at the optimum pressure corresponding to the expansion ratio for the expander (11) is fed so as to increase the efficiency of the expander (11), thereby making it possible to improve the performance of the Rankine cycle device.
