PURPOSE: To transmit information securely via the same transmission line by simple circuit constitution using a TTL integrated circuit, etc., by generating time-division channels at a transmission side and by using one channel for up transmission and the other for down transmission.

CONSTITUTION: At transmission-reception side A, n-channel multiplexer 11 is provided. Corresponding to one input terminal of multiplexer 11, a pulse of a constant voltage is sent to the output terminal of multiplexer 11 and a channel which obtains this constant voltage is used as an up channel. The other input terminal is used as that for down data and the binary voltage that corresponds to the down data is applied to multiplexer 11 by grounding respective terminals or by opening them. At transmission-reception side B, the up channel is divided into (m) different channels and a means of transmitting individual data over those different channels by superposing them upon the constant voltage is provided.