An optical property measurement apparatus 1 includes a light source unit 10, a first optical coupler 21, a second optical coupler 22, a lens 31, a lens 32, a phase modulation unit 40, a drive unit 41, an optical path length difference adjustment unit 50, a control unit 51, a light receiving unit 60, a synchronization detection unit 70, and a measurement unit 80. The phase modulation unit 40 carries out phase modulation with a frequency f. The synchronization detection unit 70 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 60, 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 51 controls the optical path length difference adjusted by the optical path length difference adjustment unit 50 to be a predetermined value based on the first signal or the second signal output from the synchronization detection unit 70.