To keep the concentration of concentrated liquid constant while following the change in raw material concentration and responding to the deterioration in capacity with the passage of time by detecting the change in the liquid level of a balance tank and controlling the pressure or the flow rate of heating steam to make evaporation correspond to the change.

When raw liquid supply is changed, the liquid level of a balance tank 5 deviates from a set level, the deviation is detected by a balance tank level controller LICA-2, the pressure of heating steam corresponding to a proper evaporation amount of a heater 2 is operated, and the set value of a heating steam pressure controller PIC-1 is renewed. When the concentration raw liquid changes, the extraction amount of concentrated liquid is changed by a concentrated liquid concentration controller DIC-1, the liquid level of a separator 3 is changed, a supply amount to the heater 2 is changed, and the amount of the raw liquid supplied to the heater 2 is changed by a controller LIC-1. In this way, the concentration of the concentrated liquid is made constant, enabling stable motion by constant amount extraction.