PURPOSE: To keep the temperature in a heat transfer surface constant all over the boiling area, by controlling the temperature by differentiating the deviation between the calorific value input by a heater and the estimated or measured heating load to a heat transfer surface corresponding to the temperature in the heat transfer surface, and by adding the differentiated value to the calorific value input by a heater, which is controlled by a feedback control based on the deviation in the temperature.

CONSTITUTION: The heat to be loaded to a boiling heat transfer surface is estimated by a processor 6 which processes the heat to be loaded to a boiling heat transfer surface, based on the temperature in the neighborhood of a boiling heat transfer surface, measured by a thermometer 3. An adder 9 calculates the deviation between the estimated load and the calorific value input by a heater, which is from P.I controller 5. The obtained deviation in calorific value is differentiated by a differentiator 7. The differentiated value is added to the calorific value inout by a heater, which is fed from the proportional and integral controller 5. Then the deviation between the calorific value input by a heater and that to be loaded to a boiling heat transfer surface can be modified prior to the change. The minute deviation between the temperature in the boiling heat transfer surface measured by the thermometer 3 and a set temperature set by a temperature setter 4 is modified by a feedback control by the proportional and integral controller 5. The temperature in a boiling heat transfer surface can be kept constant by making the control preceding to the calorific value input by a heater, to cope with the change in the calorific value loaded to the boiling heat transfer surface.