PURPOSE: To enhance the analytical preciseness of electrophoresis, by a method wherein an electrode tank used in purification is especially connected to a usual electrophoretic apparatus through a change-over cock and it is made possible to remove impurity ions in an electrolyte prior to the electrophoretic analysis by electrorefining.

CONSTITUTION: When a change-over switch 12 is brought to a state shown by a broken line and a current is supplied between both electrode tanks 10, 2, impurity ions, that is, ions having polarity same to that of a specimen ion resulting in the trough of electrophoretic analysis in a migration tube 4 are moved to the side of a leading electrode tank 2 and removed from the migration tube 4 after a definite time. Thereafter, the change-over switch 12 is changed over to a state shown by a solid line while a change-over cock 9 is changed over to fill the flowline provided to the side of a terminal electrode tank 3 wih a terminal liquid from a specimen introducing port 5 through the replacement of the terminal liquid by a usual method. In this state, when a specimen is injected from the specimen introducing port and a current is supplied between both electrode tanks 3, 2, usual electrophoretic analysis can be carried out. In this analysis, influence of imprity ions mixed in the sticking agent in a leading liquid is eliminated and analysis can be performed in high preciseness by sufficiently utilizing the function of the sticking agent (prevention of disturbance at the zone boundary surface due to heat diffusion and the stream of the liquid.