PURPOSE: To prevent the transmission trouble of a master modem by maintaining a transmission timing signal for several cycles even after the data transmission, by delaying a request-to-send signal.

CONSTITUTION: When the center terminal requests an answer signal to a terminal- side terminal, the terminal-side terminal supplies request-to-send signal RS in waveform 1 to delay circuit 3041 of the branch unit, where the trailing edge of RS is delayed by time (γ) as shown by waveform 2; and then, delay RS and transmission timing signal STI in waveform 3 from the terminal-side terminal are AND-ed by gate circuit 3051, and delay RS and transmission data SD in waveform 4 from the terminal-side terminal are AND-ed by gate 3061 respectively to obtain waveforms 5 and 6. Next, respective signals are supplied via integration circuits 307 to 309 to the master modem, where an AC modulated signal in waveform 7 is obtained and then sent to the center modem. Therefore, even after the final bit of SD from the transmission terminal, STI is maintained for several cycles and then, the final bit is modulated.