PURPOSE: To provide both function for portionwise taking a sample and function for adding the sample to the limited area of a migration gel, by inserting an electrode in the capillary of a sampling unit for electrophoresis and driving the capillary up and down.

CONSTITUTION: A capillary 3 is held in an outer cylinder 1 through an O-ring 2 and an electrode 4 is inserted in the capillary 3. The outer cylinder 1 and the electrode 4 are held by an outer cylinder holder 5. Further, a rack pinion 6 for driving the outer cylinder 1 is provided. When the sampling unit thus constituted is contacted with a sample solution, a definite amount of a sample 7 is held between the electrode 4 and the capillary 3 by a capillary phenomenon. After the sample is contacted with the surface of a migration gel 8 for a definite time in this state, the sample is allowed to rise through the outer cylinder 1. Whereupon, the capillary 3 also rises and the contact of the gel 8 with the sample is cut off and the sample 7 migrages through the gel 8.