PURPOSE: To efficiently secure privacy communication plural slave stations and to reduce the number of hardwares by ciphering only the signal corresponding to the slave station concerned to send it from a central station to the slave station with the signal sent slave station to the" central station.

CONSTITUTION: The prediction of a signal between plural stations 6 is difficult since an outgoing signal sent from the station 5 to the station 6 is ciphered by an imcoming signal sent from the slave station 6 to the central station 5 by passing through a transmission line 2 via a light passive divider 7 and changed frequently and the outgoing signal is decoded by the incoming signal in the station 6. Besides, since even at a state that the incoming signal is nearly in a non-signal state and only the least signification bit is changed, the tapping between plural slave stations is impossible by replacing the positions of the most signnificant bit and the least one. Furthermore, since the privacy communication is performed even at the time of the non-signal state, the incoming signal is scrambled, the exclusive OR of the outgoing, incoming signals is performed. Thus, the numberof the hardwares required for ciphering are reduced by using a random property of the incoming signal without generating a random pattern.