Abstract: An external cavity semiconductor laser light source which comprises: an external cavity semiconductor laser light source unit; an optical filter for selecting an output beam of the external cavity semiconductor laser light source unit in a single mode; a drive unit for varying a wavelength of a transmitted light or a wavelength of a reflected light, from the optical filter; a wavelength control unit for controlling the drive unit; an optical frequency reference light source for generating a light having a reference optical frequency; an optical frequency/voltage conversion unit for generating a signal which corresponds to a difference in optical frequency between the light outputted from the optical frequency reference light source and the output beam of the external cavity semiconductor laser light source unit; a low pass filter for transmitting a low-frequency component of the signal of the optical frequency/voltage conversion unit; a switch for feeding the signal into the external cavity semiconductor ligh