An optical unit configured to send light to an external optical system and to receive return light back from said external optical system, the external optical system including a diffraction grating and a first reflecting unit (R1a,R1b) for directing light from the optical unit towards said diffraction grating and returning light from said diffraction grating to the optical unit, the optical unit comprising: a light source for emitting a light; an optical splitting unit for splitting returning lights from the external optical system into a plurality of lights; a polarizing unit for increasing and decreasing quantity of lights transmitted therefrom corresponding to polarization state of the returning lights; and an optical receiving unit which has a plurality of photo-detecting elements (6_1 to 6_4) for detecting the lights transmitted through the polarizing unit; characterized in that: the light source, the optical splitting unit, the polarizing unit, and the optical receiving unit are unitedly integrated into one light emitting/receiving composite unit, the light emitting/receiving composite unit is configured to emit light to and receive return light from said first reflecting unit,(R1a,R1b) of said external optical system, said first reflecting unit (R1a,R1b) not being integratedly united into said light emitting/receiving composite unit, the optical unit further comprises a lens unit (8a,8_1 to 8_4) for causing the light emitted from the light source to go out from the composite unit and for converging the returning lights split by the optical splitting unit, the optical splitting unit comprises a plurality of optical splitting films and a λ/4 plate arranged between the plurality of optical splitting films, the lens unit comprises a plurality of lenses (8_1 to 8_4) each of which corresponds to a respective optical splitting film of said optical splitting unit, said plurality of lenses (8_1 to 8_4) being formed in a cover made of transparent material, and the optical splitting unit is fixed on the lens unit. a light source for emitting a light; an optical splitting unit for splitting returning lights from the external optical system into a plurality of lights; a polarizing unit for increasing and decreasing quantity of lights transmitted therefrom corresponding to polarization state of the returning lights; and an optical receiving unit which has a plurality of photo-detecting elements (6_1 to 6_4) for detecting the lights transmitted through the polarizing unit; characterized in that: the light source, the optical splitting unit, the polarizing unit, and the optical receiving unit are unitedly integrated into one light emitting/receiving composite unit, the light emitting/receiving composite unit is configured to emit light to and receive return light from said first reflecting unit,(R1a,R1b) of said external optical system, said first reflecting unit (R1a,R1b) not being integratedly united into said light emitting/receiving composite unit, the optical unit further comprises a lens unit (8a,8_1 to 8_4) for causing the light emitted from the light source to go out from the composite unit and for converging the returning lights split by the optical splitting unit, the optical splitting unit comprises a plurality of optical splitting films and a λ/4 plate arranged between the plurality of optical splitting films, the lens unit comprises a plurality of lenses (8_1 to 8_4) each of which corresponds to a respective optical splitting film of said optical splitting unit, said plurality of lenses (8_1 to 8_4) being formed in a cover made of transparent material, and the optical splitting unit is fixed on the lens unit. the light source, the optical splitting unit, the polarizing unit, and the optical receiving unit are unitedly integrated into one light emitting/receiving composite unit, the light emitting/receiving composite unit is configured to emit light to and receive return light from said first reflecting unit,(R1a,R1b) of said external optical system, said first reflecting unit (R1a,R1b) not being integratedly united into said light emitting/receiving composite unit, the optical unit further comprises a lens unit (8a,8_1 to 8_4) for causing the light emitted from the light source to go out from the composite unit and for converging the returning lights split by the optical splitting unit, the optical splitting unit comprises a plurality of optical splitting films and a λ/4 plate arranged between the plurality of optical splitting films, the lens unit comprises a plurality of lenses (8_1 to 8_4) each of which corresponds to a respective optical splitting film of said optical splitting unit, said plurality of lenses (8_1 to 8_4) being formed in a cover made of transparent material, and the optical splitting unit is fixed on the lens unit.