1. A data conversion device comprising: a storage for storing conversion data including at least one of a conversion table and interpolative calculation data; a buffer memory device capable of storing data input successively, and outputting a predetermined items of the stored data to a data processor simultaneously, and capable of temporarily storing the predetermined items of the processed data processed by the data processor, and outputting the predetermined items of the processed data; and a data processor receiving and processing simultaneously in parallel the predetermined items of the stored data output from said buffer memory device, outputting the predetermined items of the processed data simultaneously in parallel, and forming a plurality of identical conversion tables in said buffer memory device according to said conversion data stored in said storage, wherein said buffer memory device includes a memory controller being capable of allocating at least a part of a memory thereof for the conversion tables, and relating a plurality of series of data to be converted to the respective conversion tables formed in the memory so as to read converted data corresponding to the series of the data to be converted from the respective conversion tables simultaneously in parallel.