The apparatus comprises a controller (113) for supplying a constant injection current: a semiconductor laser (LD) (112) which is supplied with the injection current and outputs a laser beam; an etalon (114) for passing specific wavelength of the laser beam output from this semiconductor laser; a photodetector (PD) (116) for measuring the intensity of the beam output from the etalon; and a controller (117) which controls the temperature of the LD at first so that the output of the PD is maintained at a given value, stores the temperature of the LD when the output of the PD is maintained at the given value, and controls the temperature of the LD on the basis of the last stored temperature.
