In an optical sensing system, a phase-comparator compares the phase of an output light beam detected by a photo-detector with an input light beam as to whether those are in-phase or out-of-phase, and senses a magnitude of a physical quantity to be measured, which modulates the input light beam. The light produced by the light source is splitted into two light beam components and modulated in its light-intensity by the physical quantity. Both the light beam components are given a time difference .tau.(=(2m+1)/2f, where m=1, 2, . . . n) therebetween, are mixed again and are supplied to the photo-detector.