An optical property measurement apparatus for measuring an optical property of a sample in a flow passage using optical interference comprising: a light source unit for emitting light; a first optical coupler for branching the light emitted from the light source unit into two and outputting the light components as first branched light and second branched light; a second optical coupler for inputting the first branched light output from the first optical coupler and passed through a first branched light path, inputting the second branched light output from the first optical coupler and passed through a second branched light path and a flow passage on the second branched light path, and causing the first branched light and the second branched light thus input to interfere with each other to output interfering light; a phase modulation unit provided on the first branched light path or the second branched light path between the first optical coupler and the second optical coupler for carrying out phase modulation of the light, which is propagated on the light path, with a frequency f; an optical path length difference adjustment unit for adjusting a difference in the respective optical path lengths of the first branched light path and the second branched light path between the first optical coupler and the second optical coupler; a light receiving unit for receiving the interfering light output from the second optical coupler and outputting an electrical signal having a value corresponding to the received light intensity; a synchronization detection unit for inputting the electrical signal output from the light receiving unit, outputting a first signal having a value which corresponds to a magnitude of a component of the frequency f included in the electrical signal, and outputting a second signal having a value which corresponds to a magnitude of a component of a frequency 2f included in the electrical signal; a control unit for controlling the optical path length difference adjusted by the optical path length difference adjustment unit to be a predetermined value based on the first signal or the second signal output from the synchronization detection unit; and a measurement unit for measuring an optical property of a sample in the flow passage based on the first signal and the second signal output from the synchronization detection unit; wherein the measurement unit obtains phase difference between the first branched light path and the second branched light path on the basis of a ratio of values of the first signal and the second signal, and measures phase difference of the sample on the basis of the phase difference between the first branched light path and the second branched light path. a light source unit for emitting light; a first optical coupler for branching the light emitted from the light source unit into two and outputting the light components as first branched light and second branched light; a second optical coupler for inputting the first branched light output from the first optical coupler and passed through a first branched light path, inputting the second branched light output from the first optical coupler and passed through a second branched light path and a flow passage on the second branched light path, and causing the first branched light and the second branched light thus input to interfere with each other to output interfering light; a phase modulation unit provided on the first branched light path or the second branched light path between the first optical coupler and the second optical coupler for carrying out phase modulation of the light, which is propagated on the light path, with a frequency f; an optical path length difference adjustment unit for adjusting a difference in the respective optical path lengths of the first branched light path and the second branched light path between the first optical coupler and the second optical coupler; a light receiving unit for receiving the interfering light output from the second optical coupler and outputting an electrical signal having a value corresponding to the received light intensity; a synchronization detection unit for inputting the electrical signal output from the light receiving unit, outputting a first signal having a value which corresponds to a magnitude of a component of the frequency f included in the electrical signal, and outputting a second signal having a value which corresponds to a magnitude of a component of a frequency 2f included in the electrical signal; a control unit for controlling the optical path length difference adjusted by the optical path length difference adjustment unit to be a predetermined value based on the first signal or the second signal output from the synchronization detection unit; and a measurement unit for measuring an optical property of a sample in the flow passage based on the first signal and the second signal output from the synchronization detection unit; wherein the measurement unit obtains phase difference between the first branched light path and the second branched light path on the basis of a ratio of values of the first signal and the second signal, and measures phase difference of the sample on the basis of the phase difference between the first branched light path and the second branched light path.