Bubbles can be removed regardless of an individual difference of a pump to fill an electrophoresis medium into a capillary. Of flow passages formed between an inner side surface of a container for accommodating the electrophoresis medium and a side surface of a plunger, one of the flow passages causing an electrophoresis medium to be easily stagnant is formed to have the cross-sectional area larger than the cross-sectional area of the other flow passage on the opposite side. In other words, the flow passage portion causing the electrophoresis medium to be easily stagnant is formed in such a manner as to increase a flow amount of the electrophoresis medium. This can eliminate a region having an extremely small amount of electrophoresis medium flow in the pump.