A technique for converting a compressed XML document to a format usable by an application is provided. The technique is capable of identifying an XML document even in a state that data thereof are compressed and capable of identifying a compression type thereof. When an XML document is originally compressed, an XML header is added to the compressed document, the XML header describing an encoding declaration which indicates that the compressed XML document is provided with encoding inclusive of compression. Thereafter, in an XML parser, a decoder, which converts the XML document into a coding system used by an application, decompresses the XML document based on description of the encoding declaration in the XML header.