PURPOSE: To improve the availability of a digital signal transmission line by decreasing the number of bits to be multiplexed as the redundant bits for each frame despite the large bit length of a circuit identification pattern.

CONSTITUTION: A the transmission side, one of plural circuit identification patterns having the n-bit length (n: a positive integer and containing different numbers of '1' is divided into L pieces (L: a positive integer, n≥L). These divided patterns are successively multiplexed as the redundant bits for each frame and transmitted. At the reception side, the numbers of '1' or '0' of a circuit identification pattern reproduced by the redundant bits are counted every (n) bits. Then the circuits are identified based on the counting result. As a result, the availability of a digital signal transmission line is improved.