PURPOSE:To perform transmission without any error by detecting an ineffective data area in a transmit frame entering a node station and then carrying out the bypassing operation of a node station when the node station is bypassed. CONSTITUTION:When an ineffective data area in the transmit frame enters a multiplexing part 10 completely after a power-down signal is outputted, that is detected by an ineffective data area detecting circuit 13, which sends out a detection output signal to a switching part 12. Consequently, the switching part 12 operates and only the repeating operation of a repeating part 9 is carried out at the node station, so that the bypassing operation is completed. Consequently, data is transmitted through a transmission line 7, the repeating part 9, and a transmission line 8 and the multiplexing part 10 is bypassed. In this case, the bypassing operation is performed when ineffective data in the transmit frame enters the multiplexing part 10, so the ineffective data is erased, but necessary data remains as it is, so that the data transmission is performed accurately.
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