PURPOSE: To improve transmission efficiency by making preceding node information ineffective in case of occurrence of a transmission error and then by adding a new check code.

CONSTITUTION: A data block transmitted from preceding node A is inputted from selector circuit SEL(1) to check CHK and result R is fed back to SEL(1), thereby using the circuit as generator GEN this time. Data from the preceding node, on the other hand, are stored temporarily in buffer memory BFM and then sent out from selector circuit SEL(2) with the check result added. Without being inputted to check circuit CHK, data until the node two stages ahead are transmitted by timing control circuit CTLTIM directly from the gata circuit to next node B.