PURPOSE: To simply detect the extraction timing of an objective living body macromolecular substance without using a marker, by measuring the potential difference between a terminal electrode and a potential detecting electrode.

CONSTITUTION: In an apparatus for extracting and concentrating the living body macromolecular substance such as the DNA segment or protein contained in a gel such as agarose from said gel on the basis of an equal velocity electrophoretic theory, not only a terminal electrode 12 and a leading electrode 14 supplying a migration current are provided to both terminal parts of a migration column 2 but also a potential detecting electrode 20 is provided between the terminal electrode 12 and an extraction port 10. As a premise for an equal velocity electrophoresis, the magnitude order of the conductivity (mobility) of the terminal ion of a terminal solution and the conductivities of the leading ions of the objective living body macromolecular substance and a leading liquid is; terminal ion < objective living body macromolecular substance < leading ion. Therefore, by measuring the potential difference between the potential detecting electrode 20 and the terminal electrode 12 by a potential difference measuring device 22, the arrival of the objective living body macromolecular substance can be detected.