20. The message processing method as set forth in claim 16, wherein said message encoding step comprises the steps of: when message data is applied, examining whether a node of the parse tree being scanned is a node corresponding to a selected element and when the node is not the corresponding node, adding 1 to an index value being operated and scanning the subsequent node of the parse tree to again return to the examination whether the node being scanned corresponds to the selected element, and when the node is the corresponding node, encoding the index value being operated and encoding the element in question to output the encoding results of the index value and the element in question linked together.
16. A message processing method of sending and receiving a message composed of one or more elements between different kinds of devices and processing the message, comprising the steps of: holding a parse tree generated from syntax descriptions defining structure of a message sent or received; as message data to be processed is applied, scanning said parse tree to encode and decode a value and outputting encoded and decoded values; processing a preamble field of a message for use in indicating whether an element exists or not in message data sent or received; and processing an index field of a message for use in indicating what number of element is selected in message data whose one of a plurality of elements is selected to be sent or received.