PURPOSE: To achieve word synchronization and bit synchronization even if a synchronizing signal is used, by assigning codes by putting it to practical use that the level distribution of a difference signal is negative index distribution.

CONSTITUTION: At a transmission side, when a difference signal is at a level 0, a 0 is assigned to the 4th digit d3, and 1s to other digits and when at other levels, a 0 is assigned to the 4th digit d3 and 1s are assigned in the order of a code with less 0s; and a 1 is assigned to the digit d3 after 0s are assigned to both the digits d2 and d1, thus performing code conversion. At a reception side, the mark rate of a specific digit in a word is measured and control is exercised to obtain the digit d3 having a minimum mark rate, thereby achieving word synchronization. Thus, a word having more 1s is assigned to a level high in occurrence frequency, so bit synchronization is facilitated.