An optical communication system includes at least one optical ID tag and
at least one optical reader. Identification information is communicated
between the optical ID tag and the optical reader using light as a
communication medium. The optical ID tag includes storing units for
storing the identification information; reflecting units for reflecting
incident light in the incident direction; and modulating units for
modulating reflected light on the basis of the identification information
stored in the storing units. The optical reader includes random number
generating units for generating a random number; emitting units for
emitting light having an oscillation frequency specific to the generated
random number; photoelectric conversion units for performing
photoelectric conversion to the reflected light incident in the emitting
direction; extracting units for extracting a signal component having the
oscillation frequency from the signal resulting from the photoelectric
conversion; and demodulating units for demodulating the signal component.