The present invention has an object to provide an optical communication system which keeps the confidentiality of information at the optical level to ensure the high safety. To this end, the optical communication system according to the present invention comprises: a PMD providing apparatus that gives the polarization-mode dispersion (PMD) which is changed in accordance with a variable pattern previously determined between an optical sender and an optical receiver to an optical signal output from the optical sender; to send the optical signal to an optical line network; and a PMD restoring apparatus that gives the PMD negating the PMD provided by the PMD providing apparatus to the optical signal propagated through the optical line network and restores the optical signal to the state at the transmission time, to send it to the optical receiver.