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.
18. The message processing method as set forth in claim 16, wherein said message decoding step comprises the steps of: when message data is applied, first reading and storing a preamble field, initializing a variable indicative of a bit position in the preamble field to 0, examining whether there exists an element yet to be decoded in the message data, when no element yet to be decoded exists, outputting a decoding result of each element, when an element yet to be decoded exists, scanning the subsequent node of the parse tree to examine whether an element corresponding to the node in question is an omissible element and when the element in question is not an omissible element, decoding the element in question to again return to the examination whether an element yet to be decoded exists, when the element in question is an omissible element, if the contents of the current bit position in the preamble field indicate that "omissible element is omitted", adding 1 to a variable indicative of the current bit position to return to the examination whether an element yet to be decoded exists, and if the contents of the current bit position in the preamble field indicate that "omissible element is not omitted", decoding the element in question and adding 1 to the variable indicative of the current bit position to return to the examination whether an element yet to be decoded exists.