The troublesomeness during the setting of a plurality of capillaries is eliminated by composing pairs of electrodes, which are electrically connected to the common electrode, and capillaries. By bringing electrodes installed in the vicinity of each capillary disposed at the pitch of wells on the side of sample plate (within the area of the wells) into electrical contact with a common electrode, the capillaries and electrodes are made integral in construction. When a voltage is applied to the electrophoretic instrument via a common electrode portion, the voltage is applied to the electrodes for each capillary. This enables an inexpensive microtiter plate, etc. to be used and a multiple of capillaries to be simultaneously inserted, attached and detached.