PURPOSE: To permit efficient utilization of heat and to facilitate operation by branching the flow of raw materials to two streams, and joining the streams after introducing the streams into distilling columns which are operated respectively under different conditions then introducing the same into the final one column thereby distilling the raw materials at the highest temp. under the highest pressure.

CONSTITUTION: At least 3 units of distilling columns are operated respectively at different pressures and temps., and a distilling column 101 and a distilling column 102 are arranged in parallel. The column 101 or 102 and a distilling column 103 are arranged in series. The extracts which are raw materials are divided into two streams which are flooded in the column 101 and the column 102, respectively. After the greater part of solvents are evaporated, the fluids in the respective tower bottoms are again joined and are introduced into the column 103. The remaining solvents are distilled and separated. The fluids of the solvent vapor from the respective column tops are heat-exchanged with the raw materials to be flooded into the distilling column which is operated at low temp. and pressure. The fluids themseleves are condensed and the latent heat thereof is effectively recovered.