PURPOSE: To improve the transmission efficiency of a data communication system by transmitting a data signal with error correction code with the identical bit length with the data signal.

CONSTITUTION: Respective bits H0-H3 constructing an error correction code (h) are converted to a conversion code sdh with the identical bit length with a bit Di together with respective bits D0-D3, and transmitted from a data transmission device 1 to a data receiving device 2 as a transmission signal (s). Therefore, the throughput on a transmission path 3 is not deteriorated by the error correction code (h). Thus, the transmission efficiency of the data communication system can be improved.