PURPOSE: To decrease energy consumption while maintaining the specified concn. of the impurities in a distillate by enabling the control target values of supply rates of a heat source and a cold source according to the fluctuation of the flow rate and compsn. of a feed liquid.

CONSTITUTION: Means for detecting the flow rate 22 and compsn. 21 of a feed liquid 4 and a detector 23 for detecting the concn. of the impurities in distillate 6 are provided. The signals from the detecting means are supplied to an arithmetic unit 24. The unit 24 calculates the set value 25 of a reflux ratio and the set value 26 of a temp. controller in a column as control target values and supplies the same respectively to a reflux ratio setter 12 and a controller 11 for the temp. in the column. The unit 24 calculates these control target values in such a way that the total sum of the energy of the heat source and cold source consumed by a boiler 2 and a condenser 3 is minimized and that the concn. of the impurities to be contained in the distillate 6 is kept at a specified value or below.