PURPOSE: To discriminate the switch state of a terminal device in a short time by using the operating modulation frequency with a frequency used at a data collection center and frequencies for the number of shares of switches used on one terminal device to decrease the entire frequencies.

CONSTITUTION: A control section CONT of a system discriminating the switch state of subscribers of optical communication collects data and transmits transmission information D0 to a transmitter Tx at each prescribed period. The information D0 is modulated at the transmitter Tx by the 1st modulation frequency f1 and transmitted from an infrared light emitting diode LED. Further, each terminal device (subscriber) is incorporated with transceivers TR1∼TRn and switches S1a∼Snc. The transceivers TR1∼TRn receives the information of the modulation frequency f1 from the transmitter Tx, a counter is started, the signal is modulated in response to the state of the switches S1a∼Snc, the frequency corresponding to a depressed switch in the switches S1a∼Snc is transmitted to discriminate the switch state of the terminal device in a short time.