PURPOSE: To perform character conversion at each field, by referring a category table for an arbitrary input code through designation, in a data processor.

CONSTITUTION: A character type designation control byte A is provided for each field and classified into a category designation 1(B) and a cataegory designation 2(C). The type of input characters is designated at the cataegory designation 1(B), and special conversion designation is made at the cataegory designation 2(C) and the output code conversion is made with an output table corresponded through the reference of a category table. (D). Thus, the field of document is read with a data processor and the conversion is easily done at the output point of time.