PURPOSE: To separate and detect objective components accurately, by a method wherein the objective components migrated and separated through a detector are trapped in the middle of a cataphoresis tube, then the electrolyte is replaced with new one, and polarity of applied voltage is switched for migrating the objective components in reverse direction.

CONSTITUTION: Electrode baths 3 and 4 are connected to both ends of a high voltage power supply circuit 2, respectively. A sample inlet 5, a detector 6, and a six-way cock 7 are connected to the line 8 through 11, in order, between the electrode baths 3 and 4. Sample is poured from the sample inlet 5 for performing the equal velocity cataphoresis. The detector 6 detects the objective components. After the sample poured has entered into a trap tube 12, the six-way cock 7 is switched so as to stop the current. Drain cocks 13 and 14 are opened for draining electrolyte except for that gathered in the trap 12. After that, new electrolyte is supplied. Then, the six-way cock 7 is returned to the original position for reversing the polarity of the high voltage power supply circuit 2, and performing cataphoresis of the objective components in reverse direction, so that the accurate separation can be provided.