To correctly execute the learning of a light receiver even when the distance between the light receiver and a light transmitter is large by adopting a reception reproducing teacher signal as a teacher signal so as to make the relative phase of the teacher signal normally coincide with the relative phase of a reception learning signal.

Signals having mutually different speeds by the repetition pattern of one and zero are generated as the wavelength-corresponding teacher signal from a teacher signal generator 31 on the transmission side so as to transmit wavelength-multiplexed light. On the reception side, wavelength-multiplexed light from an optical fiber 1 is branched and transduced into an electric signal by a photodetector 45. Then, the basic frequency components of the respective teaching signals are separated by filters 461-464 and supplied to a learning circuit 23 as the teacher signal of the one and zero pattern. The other branced light is wavelength-separated, transduced into the electric signal by photodetector array 7 and supplied to a neural network 9.