Abstract: In an apparatus for gel electrophoresis in which a sample of fluorophore-labelled DNA fragments is caused to migrate by electrophoresis through a gel electrolyte layer in an electrophoresis plate from top to bottom, thereby separating the sample into individual DNA fragments, and a laser beam is launched horizontally into said electrolyte layer from one side of the electrophoresis plate in a direction perpendicular to the longitudinal axis of said electrophoresis plate, with the emitted fluorescences being detected to determine the base sequences of the respective DNAS, a mirror for reflecting fluorescences is provided at the back of the electrophoresis plate in such a way that it is parallel to the direction of laser beam application, and the fluorescences reflected by this mirror are received by a fluorescence detector such as a CCD sensor or MOS linear image sensor. In one embodiment, the mirror is inclined by an angle of 45.degree.