PURPOSE: To raise accuracy of temperature control of a distillation column and conduct a stabilized operation, by controlling steam flow rate for heating a reboiler by measuring a flow rate of steam fed to a reboiler to heat it and temperatures of steam of liquid in the reboiler.

CONSTITUTION: A distillation column 1 is provided with a reboiler 3 through which a part of vapor of boiling liquid in said column is naturally circulated. Heating steam is supplied to the reboiler 3 from a boiler 4, the flow rate of which is controlled by a steam flow rate control valve 6. On the other hand, a temperature sensor 7 to detect vapor temperatures of boiling liquid is provided in a vapor circulation passage and a flow rate sensor 5 is provided in a generated steam passage from a boiler 4, the output of the flow rate sensor 5 and that of the temperature sensor 7 being connected to a control circuit 8. The feed rate of heating steam is controlled as a function of steam temperatures by said control circuit 8 to open and close the steam flow rate control valve 6.