PURPOSE: To improve the thickening and separating performance of a migration layer by forming a part having a high salt concn. and small sectional area to part of a column type electrophorectic cell.

CONSTITUTION: A preceding liquid is supplied into a migration cell 9 from an upper supply port 11 for the migration cell liquid and the succeeding liquid from a lower supply port 12 for the migration cell liquid. A sample soln. is supplied into a cell from a sample injection port 13. Electrophoresis is then executed by passing electric current between an upper electrode 1 and a lower electrode 2 while the discharge liquid of the migration layer having the ion concn. higher than the ion concn. of the preceding and succeeding liquid is continuously run from a supply port 3 for the discharge liquid of the migration layer through a small-diameter pipe part 5 to a discharge port 4 for the migration layer. Then the migration layer which is separated from the sample and ascends in the cell 9 is thickened in the discharge liquid of the migration layer having the high ion concn. and is discharged from the port 4 to the outside. Since the ion migration speed in the part where the salt concn. is high is increased, the influence of diffusion is decreased and the sepn. performance is improved. The dispersion concn. is increased as well.