A light-receiving/emitting composite unit comprising: a light source for emitting coherent light; a polarizing beam splitter for splitting the light emitted from the light source into two light beams (La1, La2) having different polarized components, then radiating the two light beams (La1, La2) to an external optical system (11, 13, 14), and combining the two light beams (Lb1, Lb2) reflected from the external optical system (11, 13, 14) to generate combined light (Ld); a phase plate; light splitting means for splitting the combined light (Ld) generated by the polarizing beam splitter into plural coherent light beams (Ldl, Ld2, Ld3, Ld4); polarization means (42; 42_1, 42_2, 42_3, 42_4) for transmitting only a predetermined polarized component of the split combined light (Ld; Ld1, Ld2, Ld3, Ld4); and light receiving means (52; 52_1, 52_2, 52_3, 52_4) for photoelectrically converting coherent light transmitted through the polarization means (42; 42_1, 42_2, 42_3, 42_4) and thus generating an interference signal, wherein that said phase plate is arranged between the light source and the polarizing beam splitter and in that it is adapted for changing the polarization state of the light emitted from the light source and leading the light to the polarizing beam splitter characterized in that the phase plate is provided between the light splitting means and the polarization means (42; 42_1, 42_2, 42_3, 42_4) and the phase plate changes the polarization state of the split combined light (Ld; Ld1, Ld2, Ld3, Ld4) and leads the split combined light (Ld; Ld1, Ld2, Ld3, Ld4) to the polarization means (42; 42_1, 42_2, 42_3, 42_4); and the light splitting means, the phase plate and the polarization means (42; 42_1, 42_2, 42_3, 42_4) are sequentially stacked. a light source for emitting coherent light; a polarizing beam splitter for splitting the light emitted from the light source into two light beams (La1, La2) having different polarized components, then radiating the two light beams (La1, La2) to an external optical system (11, 13, 14), and combining the two light beams (Lb1, Lb2) reflected from the external optical system (11, 13, 14) to generate combined light (Ld); a phase plate; light splitting means for splitting the combined light (Ld) generated by the polarizing beam splitter into plural coherent light beams (Ldl, Ld2, Ld3, Ld4); polarization means (42; 42_1, 42_2, 42_3, 42_4) for transmitting only a predetermined polarized component of the split combined light (Ld; Ld1, Ld2, Ld3, Ld4); and light receiving means (52; 52_1, 52_2, 52_3, 52_4) for photoelectrically converting coherent light transmitted through the polarization means (42; 42_1, 42_2, 42_3, 42_4) and thus generating an interference signal, wherein that said phase plate is arranged between the light source and the polarizing beam splitter and in that it is adapted for changing the polarization state of the light emitted from the light source and leading the light to the polarizing beam splitter characterized in that the phase plate is provided between the light splitting means and the polarization means (42; 42_1, 42_2, 42_3, 42_4) and the phase plate changes the polarization state of the split combined light (Ld; Ld1, Ld2, Ld3, Ld4) and leads the split combined light (Ld; Ld1, Ld2, Ld3, Ld4) to the polarization means (42; 42_1, 42_2, 42_3, 42_4); and the light splitting means, the phase plate and the polarization means (42; 42_1, 42_2, 42_3, 42_4) are sequentially stacked.