PURPOSE: To favorably discharge the precipitate formed at the phase separation boundary region surface in a pulsating liquid extraction column without stopping the function of the column.

CONSTITUTION: The precipitate is discharged by an extraction pipe 14 opened at the level of the phase separation boundary region surface 10 in a lower decanter 6. A replenishing pipe 16 replenishes a heavy phase simultaneously therewith. The height of the phase separation boundary region surface 10 is detected by two air bubble passage pipes 20 and 22. A controller 26 controls regulating valves 24 and 48 in accordance with the result of the detection by these air bubble passage pipes. These regulating valves maintain the height of the phase separation boundary region surface 10 by regulating the inflow rate and discharge rate of the heavy phase 4.