PURPOSE: To improve sensitivity at a reception side by sending light signals of two wavelength to one and the same light transmission path after modulating them by electric signals with phases opposite to each other at a transmission side, and performing differential processing by detecting them at every wavelength at the reception side.

CONSTITUTION: Light sources 13 and 14 output optical carrier signals with wavelength λ1 and λ2, respectively. Light with the wavelength λ1 and λ2 are demultiplexed equally by optical demultiplexers 15 and 16, respectively, and inputted to optical switches 17 and 19, and 18 and 20. The optical switches 17, 18, and 19, 20 are controlled by a differential output signal corresponding to the input electrical signals of driving circuits 11 and 12, respectively, and the optical switches 17, 18 and 19, 20 are operated with negative phases. The output light of the optical switches 17 and 18 are coupled by an optical coupler circuit 21. Similarly, two optical carrier signals are demodulated and wavelength- multiplexed with the negative phase by an optical coupler circuit 22. A light reception circuit 3 demultiplexes light components with the wavelength λm and λm by a wavelength separation circuit 31, and converts them to the electric signals by two optical detectors, then, inputs them to a differential circuit 33.