PURPOSE: To simplify the data conversion processing and to save the memory capacity by avoiding the data conversion processing to meaningless bits.

CONSTITUTION: A data conversion table 12 representing each bit location of significant bits and its item number in a data frame 11 in cross reference is generated in advance, only the data of the bit location represented thereat is extracted from the data frame 11 and an item number corresponding to the data is extracted from the data conversion table 12 when the data is a prescribed value. Thus, only significant bits designated by the data conversion table 12 are selected from the bits in the data frame 11, on data conversion is implemented when the data is a prescribed value and meaningless bits are processed. Thus, the data conversion processing is simplified and the memory capacity is saved.