PURPOSE: To monitor data at an end/end of B-channel data by inserting a frame synchronization pattern after parity check in place of parity bits and monitoring the pattern.

CONSTITUTION: Transmission data with one bit parity added thereto from a terminal equipment 4 are fed to a communication equipment 12, and when a parity check circuit 10 does not detect any parity error, a frame transmission circuit 9 inserts a frame synchronization signal having a synchronization pattern in place of the parity bit. Then the frame synchronization is monitored by a frame synchronization monitor circuit to monitor data at end/end of the B-channel data. Furthermore, a parity addition circuit 8 adds the parity bit to the reception data in place of the synchronization frame and the data are utilized effectively.