To provide optical dispersion monitoring method and apparatus capable of precisely monitoring dispersion with a simple constitution and to provide an optical transmission system using the same.

The optical dispersion monitoring apparatus comprises a light receiving part 10 for converting an inputted optical signal to an electric signal VIN, a signal change position detection part 20 for detecting voltage levels VX at rise and fall cross points of an output signal waveform from the part 10, and a cumulative dispersion information extraction part 30 for comparing voltage levels VX at cross points with a reference signal VREF to extract cumulative dispersion information.