PURPOSE: To reduce the electric power for transmission, by connecting a light emitting element properly to plural pieces of optical waveguides corresponding to the number of bits, and obtaining an optical digital signal by the combination of the light emitted by the light emitting element.

CONSTITUTION: An optical analog signal is converted to an optical digital signal by an optical analog/electrical binary convertion part (OA/ED) 3 which converts said signal to an electric binary signal and an optical priority encoder 4 which converts the binary signal from the part 3 to an optical binary digital signal. Further, the optical digital signal is converted to an electric binary digital signal by an opto/electric conversion part 5.