Abstract: A DNA base sequencer including a gel electrophoretic means having tracks for electrophoresing fluorophore-labelled DNA fragments, a laser diode as a light source for illuminating said tracks with exciting laser light and a CCD sensor for detecting the fluorescence emitted from the illuminated DNA fragments, the laser diode has a control unit including a Peltier device for controlling the temperature of the laser diode, a Peltier device temperature setting generating means having a processor and a memory, and a temperature control circuit that generates a drive current to the Peltier device for controlling its temperature. The CCD sensor receives part of the exciting light from the laser diode as stray light and detects its wavelength.