In a coding system wherein an error correction/detect-ion 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 detect-ion 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 a 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.