1. A data processing method for processing a data file described in a markup language which describes a semantic structure of data with marks and describes information about a character code used for description of the data file into a header of the data file, the data file comprising a data portion and the header, said data processing method comprising the steps of: providing encoding with respect to the data portion header of the data file after reading the data file into a processor device; and writing information concerning the encoding in a position within the header where the information about the character code is to be written, in a format common to the information about the character code.