PURPOSE: To shorten the total polling time, by constituting that each terminal unit in the group can deliver the response data signal to the center after the detection of group polling signal and respective different time.

CONSTITUTION: The group polling signal from the center I enters the terminal signal processor 26. When group polling, the time measurement starts from the time point and after the time obtained based on the specific address register 27 to the terminal, the channel data from the channel selector 28 is fed to the upward signal transmitter 29 and delivers it to the center I. The upward signal is demodulated at the data signal demodulator 18 and then is fed to the communication signal data processor, and the data is input to the CPU16.