A double-wavelength light source unit capable of stable signal detection, is realized.The double-wavelength light source unit comprises a double-wavelength light source, a photodetector and a double-wavelength diffraction element, wherein the double-wavelength diffraction element comprises a diffraction grating which transmits a light beam of .lamda..sub.1 and diffracts a light beam of .lamda..sub.2 among the light beams of wavelengths .lamda..sub.1 and .lamda..sub.2 (.lamda..sub.1.noteq..lamda..sub.2) emitted from two laser diode diodes as the double-wavelength light source, a diffraction grating which diffracts a light beam of .lamda..sub.1 and transmits a light beam of .lamda..sub.2, a diffraction grating constituted by a grating-material forming a grating and a filler which transmits a light beam of .lamda..sub.1 since the refractive indices of both of these materials are equal at .lamda..sub.1 and diffracts a light beam of .lamda..sub.2 since the refractive indices are different at .lamda..sub.2, and a diffraction grating which transmits a light beam of .lamda..sub.1 having a first polarization plane and diffracts a light beam of .lamda..sub.1 having a polarization plane perpendicular to the first polarization plane.