A small-sized, inexpensive light wavelength measuring apparatus which has no mechanical movable sections but which is capable of high-speed measurement and high measurement precision to improve in reliability. In the light wavelength measuring apparatus, the intensity of incoming light is modulated by a light modulator in response to a reference signal supplied from a signal source and the modulation light is provided with a delay corresponding to the wavelength thereof through a group delay dispersion medium. The light is then converted into an electrical signal by a photoelectric converter, and a phase difference between the electrical signal and reference signal is obtained by a phase comparator, thereby measuring the wavelength of the incoming light.