PURPOSE: To proceed to next process without disturbing molecules eluded and held by membrane by piling an upper buffer liquid room on the top of a lower buffer liquid room which each holds a certain amount of eluded liquid and applying proper electric field to this eluded liquid.

CONSTITUTION: An upper buffer liquid room 10 is piled on the top of a lower buffer liquid room 12 which each holds a certain amount of eluded liquid and proper electric field is impressed to this eluded liquid. The current flows through the eluded liquid in the buffer rooms 10 and 12, makes the desired molecule in a sample gel piece 30 go from the piece 30 through the lower end of the sample holder 28 and a condenser 26 and elude to its lower part membrane 38. The membrane 38 holds the molecules eluded by a method proper for directly processing in the next. As the upper part of the condenser 26 does not recontrol and reorientate the condenser 26, next process is made possible without disturbing the molecules held by the membrane 38 and eluded.