PURPOSE: To provide an error correction function to an SDH signal in the concatenation mode by applying error correction arithmetic operation to data of each string of pay load so as to generate an error correction code thereby inserting the code to a stuff field of a succeeding string.

CONSTITUTION: A generating means 3 applies error correction arithmetic operation to data of each string of pay load to generate an error correction code and an insert means 4 inserts the error correction code to, e.g. a stuff field of a succeeding string. A section overhead SOH insert means 5 inserts control information to generate an SOH frame. The generating means 3 applies parallel error correction arithmetic operation to data of plural strings of pay load to generate an error correction code, then the insert means 4 inserts the plural error correction codes to a relevant stuff field in parallel. Thus, the error correction code is inserted to an SDH signal in the concatenation mode in compliance with the Modified G.709.