The present invention provides a method for determining whether or not a capillary filled with an electrophoretic medium is suitably used for electrophoresis before electrophoresis is performed using the analytes. The method comprises (a) applying an alternating-current voltage between a first electrode which is in contact with a first electrolyte solution in which one end of the capillary is immersed and a second electrode which is in contact with a second electrolyte solution in which the other end of the capillary is immersed to measure an electric conductivity of the electrophoresis medium with which an inside of the capillary is filled; and (b) determining that the capillary filled with the electrophoresis medium fails to be used suitably for the electrophoresis, when the electric conductivity is more than 4.2 mS/cm.