Abstract: An original data stream encoded into an M.sup.2 coded data stream having a plurality of bit cells has a synchronizing signal with a unique sync pattern based on the logic levels of the original data stream inserted into the M.sup.2 coded data stream. The unique pattern forming the synchronizing signal is constructed so as not to appear as a pattern in the combined data stream other that at the locations of the inserted synchronizing signal. As a result, random M.sup.2 coded data patterns are not erroneously detected to be a sync pattern, thereby avoiding a synchronization shift and the occurrence of a synchronization error. Accordingly, a correct and stable synchronization can be established so as to enable reproduction of an original data stream with high fidelity.