PURPOSE: To perform bidirectional data conversion, by providing one code conversion table and its index table at least and accessing the index table directly by input data.

CONSTITUTION: The first and the second code conversion tables 5 and 6 for data conversion and a data converter 2' consisting of an index table 4 for these tables 5 and 6 are provided between a buffer memory 1 and a shift register 3. The table 4 is addressed by the first input data of the first byte to read out the start address for tables 5 and 6 and data length information. In the table 5, data are arranged in order of contents of the first input data of the first byte and are stored in accordance with addresses stored in the table. In the table 6, data are arranged in order by a length from respective start addresses and are stored in accordance with addresses stored in the table 4. Consequently, two 8-bit code data inputs can be converted to one 16-bit code data.