23. A computer readable memory storing a control program which controls a message processing device for sending and receiving a message composed of one or more elements between different kinds of devices, said control program 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.
23. A computer readable memory storing a control program which controls a message processing device for sending and receiving a message composed of one or more elements between different kinds of devices, said control program 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. 26. The computer readable program as set forth in claim 23, wherein said control program in said preamble processing step, stores the parse tree in a continuos region on a storage region, and for decoding a message, when message data is applied, first reads and stores a preamble field, initializes a variable indicative of a bit position in the preamble field to 0, stores an address of a node being currently scanned in a variable indicative of an address of a node of the parse tree, examines whether there exists an element yet to be decoded in the message data, when no element yet to be decoded exists, outputs a decoding result of each element, when an element yet to be decoded exists, examines whether the element in question is an omissible element and when the element in question is not an omissible element, stores an address of a subsequent node in the variable indicative of an address of a node of the parse tree, scans the node of the parse tree having the address in question and decodes the element in question to 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", stores an address of a subsequent node in the variable indicative of an address of a node of the parse tree and adds 1 to a variable indicative of the current bit position to return to the examination whether an element yet to be decoded exists, and when the contents of the current bit position in the preamble field indicate that "omissible element is not omitted", stores an address of a subsequent node in the variable indicative of an address of a node of the parse tree, scans the node of the parse tree having the address in question, decodes the element in question and adds 1 to the variable indicative of the current bit position to return to the examination whether an element yet to be decoded exists.