in encryption transmission employing a conventional optical transmission apparatus , since a dedicated hardware unit to be used executes a complicated signal process , there are the problems that a high - speed transmission speed is difficult to realize and an entirety of a transmission system employing encryption becomes high in cost . an optical signal transmitter for encryption and an optical signal receiver for encryption , employing optical multi - value transmission which is high in cost and in which an effective improvement of transmission speed is difficult , are used only for exchanging an encryption key and data to be actually transmitted is transmitted by another line . a data signal is transmitted by using : the exchanged encryption key after communication of the encryption key is executed by using the transmitter and receiver for encryption transmission prior to the data transmission ; and another system of high - speed signal transmission line in an encryption transmission circuit .