An optical property measurement apparatus includes a light source unit, a first optical coupler, a second optical coupler, a lens, a lens, a phase modulation unit, a drive unit, an optical path length difference adjustment unit, a control unit, a light receiving unit, a synchronization detection unit, and a measurement unit. The phase modulation unit carries out phase modulation with a frequency f. The synchronization detection unit outputs a first signal having a value corresponding to a magnitude of a component of the frequency f included in an electrical signal output from the light receiving unit, and also outputs a second signal having a value corresponding to a magnitude of a component of the frequency 2f included in the electrical signal. The control unit controls 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.