[Problem] To prevent light reflected by a light-emitting unit from returning to a
light source and affecting the
laser oscillation frequency of the
light source. [Solution] This light-receiving device comprises: a measurement optical
system that generates measurement light having a frequency that shifts at least discretely to two or more levels; and a light
processing unit that causes prescribed oscillation light and return light of the measurement light from an object being measured to interfere with each other. The measurement optical
system has a light-emitting unit that transmits the measurement light. The measurement optical
system is also capable of transmitting the measurement light through
laser resonance caused by at least inner-
surface reflection of the light-emitting unit.