A quantum-cryptographic communication system for quantum-cryptographic communication in an optical network, including a transmitter for transmitting a packet signal having a light pulse train representing an address and a single photon pulse train for quantum cryptography, and a router including a header analyzer for extracting the address information from the light pulse train of the packet signal and a gate switch for selecting one of the optical fibers. The router routes the packet signal by selecting an optical fiber used for the next transmission path according to the extracted address information by the header analyzer and by switching the path to the selected optical fiber by the gate switch.