PURPOSE: To correct the fluctuation in the load of a sea water desalting device quickly and to stabilize the output, by providing an independent control system for a heat generation system and controlling a heat utilization system and the heat generation system discretely.

CONSTITUTION: The temp. of the high temp. water flowing through a piping system 5 for circulating water is measured with a detector 19 for outlet temp. and the output thereof is put into a temp. indicating controller 20 by which the temp. is compared and calculated with a preset value. The opening of a control valve 16 for the flow rate of bleeding is controlled by the output from the controller 20. The outlet temp. of the circulating water is maintained at a prescribed value as the inflow rate of bleeding to an intermediate indirect heat exchanger 3 is increased or decreased by the opening or closing of the valve 16. The circulating water is supplied through a flash tank 6 and a piping system 8 for circulating steam to the brine heater 9 of a desalting device 17 for sea water. Since the outlet temp. is kept constant in a heat generation system C, the circulating steam is supplied at a specified temp. and the fluctuation in the load of the heater 9 is suppressed thoroughly only by controlling the opening of a control valve 15 for the flow rate of steam.