Abstract: An electrophoretic apparatus which is equipped with means for detecting that a zone of a certain kind of ion component passes through a predetermined position of a migration tube so that the electrophoresis is divided into two stages to effect a pre-separation, i.e., a rough separation of target components and to effect a fine separation for finely separating said target component and so that the switch from the pre-separation to the fine separation in automatically effected in accordance with the output of the aforementioned passage detecting means, which can shorten an analysis time and ensure a precise quantitative or qualitative analysis.