A light dispersion measuring apparatus, provided with a light source for generating reference light pulses of short duration and of a constant frequency and also variable-frequency light pulses in synchronization with the reference light pulses but at a frequency different therefrom. A variable optical delay line delays the reference light pulses. An optical transmission medium, to be detected and to which the variable-frequency light pulses are applied is provided. An optical non-linear effect element receiver the output light pulses from the variable optical delay line and the optical transmission medium and is disposed so that when the two light pulses coincide emitted from the light source with each other, the sum component of the frequencies of the two types of light pulses produces a maximum output. A photo detector detects the sum component. Light dispersion in the optical transmission medium being measured from the delay of the variable optical delay line is adjusted so that the sum components yields a maximum output with respect to the frequency of the variable-frequency light pulses. The frequency of the reference light pulses can be made to be equal to the variable-frequency of the variable-frequency light pulses.