Provided is an electrophoresis device that, by electrophoresis, feeds a sample into capillaries and optically detects the sample, the electrophoresis device being provided with capillaries, a capillary head provided at the distal end of the capillaries, a phoretic medium-filled container used for electrophoresis and filled with a phoretic medium, a guide member that covers the side surface of the phoretic medium-filled container, a seal member that seals from below the phoretic medium filled in the phoretic medium-filled container, and a plunger that presses the seal member.