The present invention provides a light emitting/receiving composite unit (1) for emitting and receiving lights, which includes a light source (3), an optical splitting unit (4) for splitting returning lights to the composite unit (1) into a plurality of lights, a polarizing unit (5) for increasing and decreasing quantity of lights transmitted therefrom corresponding to polarization state of the returning lights, and an optical receiving unit (6) which has a plurality of photo-detecting elements (6—1 to 6—4) for detecting the lights transmitted through the polarizing unit (5), which are unitedly integrated into one unit. Also, the present invention provides a displacement detection apparatus of grating interference type, which includes thus unitedly integrated composite unit (1), and an external optical system of an objective unit having a reflection type diffraction grating RG, a reflecting unit (R1a, R1b) for reflecting lights from the composite unit (1) to the reflection type diffraction grating RG, a polarizing unit (WP1a, WP1b) for changing polarization state of diffracted lights from the reflection type diffraction grating RG, and a reflecting unit (R2a, R2b) for reflecting lights transmitted through the polarizing unit (WP1a, WP1b) to cause the reflected lights go backward.