A measuring system of transmission characteristics comprises a reference signal light generator for generating reference signal light of a reference wavelength optically intensity-modulated at a reference frequency, a measuring signal light generator for generating measuring signal light of a measuring wavelength optically intensity-modulated at a measuring frequency synchronizing with the reference frequency, a multiplexer for multiplexing the reference signal light and measuring signal light respectively generated by the reference signal light generator and measuring signal light generator and outputting them onto a to-be-measured optical line, a photodetector for converting the lights propagated on the to-be-measured line into electric signals, a reference frequency extractor for extracting the component of the reference frequency from the output of the photodetector, and a phase detector for detecting a phase of a frequency component corresponding to the measuring frequency contained in the output of the photodetector according to the output of the reference frequency extractor.