PURPOSE: To provide a gel temperature control device for DNA automatic sequencer in which temperature distribution on a glass plate is uniform and temperature accuracy is high and with which a separated image excellent in reproducibility can be obtained in a short time.

CONSTITUTION: The DNA automatic sequencer is provided with an electrophoresis tank composed of a gel 1 and two transparent glass plates 2, 3 interposing this gel 1, and an optical scanning device 6 which detects a position for indicating a movement degree of a fluorescence coloring gene sample S formed on the gel 1 through irradiation of laser beams from the outside of the glass plate 2 or 3 to detect fluorescent excitation light thereof. A cooling device 10 to be fitted in close contact with surfaces of the transparent glass plates 2, 3 of the DNA automatic sequencer is provided with a temperature control plate 11, a Peltier element 12 fitted in close contact with the rear face thereof, a heat radiation fin 13 fitted to a heat radiation side of this Peltier element 12, heat radiation fans 14 for radiating heat of this heat radiation fin 13, and a control device for controlling temperature of the Peltier element 12.