An electrophoresis device has: a sample tray (112) on which there are placed a positive-electrode-side buffer solution container (103) containing a buffer solution and a phoresis medium container (102) containing a phoresis medium, and which is driven in a vertical direction and a horizontal direction; a thermostat oven unit (113) that holds a capillary array having a capillary head in which a plurality of capillaries are bundled in a single unit at one end thereof in a state where the capillary array being held in a state in which the capillary head protrudes downward, and that keeps the interior temperature constant; a solution-delivering mechanism (106) for delivering the phoresis medium in the phoresis medium container to the capillary array from the capillary head; and a power source for applying a voltage to both ends of the capillary array. Holes for insertion of the capillary head are provided in upper sections of the positive-electrode-side buffer solution container and the phoresis medium container. The thermostat oven unit is provided with a first lid member (207) that is positioned above the sample tray and seals the upper section of the positive-electrode-side buffer solution container while the phoresis medium is being delivered by the solution-delivering mechanism.