PURPOSE: To allow plural receiving stations to receive independently and simultaneously transmit data sent out of a transmitting station to a common transmission line in response to the call signal of a control station by connecting one common transmission line of bus structure to the control station.

CONSTITUTION: A transmitted synchronizing signal is received by each station. Namely, a bit synchronizing signal is extracted by a bit synchronism extracting circuit 42 from the serial signal of the common transmission line 20 and the serial signal is converted into a parallel signal by a series-parallel converting circuit 41. Then, a synchronizing signal detecting circuit 43 detects the synchronizing signal. When the synchronizing signal is detected, the No.1 station address code (No.1 station call signal SA1) of a register 35 is led to a parallel- series converting circuit 31 through a multiplexer 34 to transmit the No.1 station call signal SA1 to the common transmission line 20. Then, the No.1 station ST1 receiving the SA1 from the common transmission line 20 transmits its station data DATA1 to the common transmission line 20 as a serial signal.