PURPOSE: To dispense with a complicated and expensive device by making a transmitting side transmit a frequency, corresponding to respective terminal, by time division, and making a receiving side convert a signal having passed through a frequency band pass filter, corresponding to the respective terminal, into a digital signal.

CONSTITUTION: The data of ON or OFF of the respective terminals 1∼4 is repeatedly transmitted to respective transmission lines 23a∼23d only during definite time, assignated to the respective terminals, by a time division timing generator 21. Then, if the data is ON, the signals of frequencies, set to the respective terminals 1∼4, are transmitted through the transmission line 22 by frequency generators 5∼8, and are separated into every frequency by frequency separators 9∼12, corresponding to receiving side terminals, and the data, transmitted by the time division, is converted into a continuous signal by one shot pulse generators 13∼16. Thus, a complicated circuit is not necessitated, and the device itself can be formed at a low price.