PURPOSE:To secure the code conversion through a constant procedure irrespective of the shift stage and the unit of the external code series by providing the code conversion memory. CONSTITUTION:Code conversion memory comprising code conversion table 51 at shift code part (a) and external code part (b) each and shift mode control part 52 accompanied to table 51 is provided to the computer or within the memory unit of the control unit. Table head address TA and internal code 16-notation number INC are selected through switch circuit 1 and 2 to be added together (3), and the addition result is set to address register 4 in the form of reference address A within table 51. Then corresponding code SC within table 51 is read out based on the contents A, and whether the SC is the 16-notation number (FF) or not is decided (6). If not, whether the SC is the 16-notation number (00) or not is decided (6). And if not, whether the SC coincides with the contents of part 52 is decided (6). Still if not, the switching of the shift mode is required. So that the SC is memorized in part 52 in the form of control data SM.