A quantum cryptography communication system includes a first data communication unit; a second data communication unit connected with the first data communication unit by a first optical fiber; and a third data communication unit connected with the second data communication unit by a second optical fiber. A first shared key is generated in the first data communication unit and the second data communication unit, and a second shared key is generated in the second data communication unit and the third data communication unit. The second data communication unit encrypts the first shared key by using the second shared key and then transmits the encrypted first shared key to the third data communication unit on the second optical fiber, and the third data communication unit decrypts the encrypted first shared key by using the second shared key to reproduce the first shared key.