PURPOSE: To stably drive a brine heater for the automatic operation of a plant for the desalination of sea water, by automatically controlling the temp. of steam to be sent to the brine heater at a value obtd. by adding a proper degree of overheating to a saturation temp.

CONSTITUTION: Pressure inside a brine heater 2 is detected, and the signal of the detected pressure is sent to a pressure-temp. converter 14 and converted into the signal of a saturation temp. corresponding to said pressure. Then, the signal of a temp. as a value obtd. by adding a proper degree of overheating to said saturation temp. is outputted to a discharge regulator valve 9 to control said valve. Thus, a temp. regulator is automatically controlled without hands, and the plant for the desalination of sea water is operated under a stabilized condition.