In a coding system wherein an error correction/detection coding is combined with a synchronization recovering technique using a synchronization code, the problems of a pseudo synchronization and a step out due to error detection are solved. There is provided a coding part for coding an input multiplexed code string to an error correcting/detecting code comprising an information bit and a check bit, and code string assembling part for inserting a synchronization code into any one of a plurality of periodically predetermined synchronization code inserting positions in the code string, for arranging the information bit at an optional position in the code string, and for arranging the check bit at a position other than the synchronization code inserting positions in the code string to assemble an output code string.