Abstract: An injection current controller 113 is provided to supply a given injection current. A semiconductor laser (LD) 112 receives the injection current from the injection current controller to output a laser beam. An etalon 114 discriminates the wavelength of the laser beam output from the semiconductor laser. A photodetector (PD) 116 is provided to detect the intensity of the output beam from the etalon. An LD temperature controller 117 performs temperature control of LD so as to bring the output of PD to a predetermined value and, when the output of PD becomes the predetermined value, reserves a corresponding temperature of LD and, at the next temperature control start time, initiates temperature control based on a finally reserved temperature of LD.