A wavelength filter is disposed between a laser diode and a photodiode that receives the rear-facet output light of the laser diode. Either a part on one side of an input or an output surface of the wavelength filter is bounded to a protrusion provided in a metal holder for holding the wavelength filter. As a result, a support structure is obtained that does not bring a damage on the wavelength filter caused by degradation of optical characteristics or temperature variation of the wavelength filter when support-fixing a wavelength filter having a complex refractive index crystal.