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.