1. A protocol encoding device for use in encoding an abstract syntax for an application layer in Open Systems Interconnection (OSI) into a transfer syntax for a presentation layer in OSI, said abstract syntax being a set of data types described by an abstract syntax Notation One (ASN. 1), said abstract syntax representing a data structure which is defined by the data types in the ASN.1 and data values corresponding to the data types in the ASN.1, said transfer syntax being a protocol data unit (PDU) having said data values, said protocol encoding device comprising: an abstract syntax storage for storing said abstract syntax therein; a structure generating unit, connected to said abstract syntax storage, for automatically generating, on the basis of a predetermined conversion rule, structures corresponding to the data structure represented by said abstract syntax, said structures being defined by data types in C language, said predetermined conversion rule defining a conversion from the data types in the ASN.1 to data types in C language; a structure storage, connected to said structure generating unit, for storing said structures therein; a value tree list (VTL) generating unit, connected to said structure storage, for automatically generating a VTL on the basis of the data values stored in said structure storage, the VTL being a table indicative of said data values; an ASN.1 syntax tree (AST) generating unit, connected to said abstract syntax storage, for automatically generating an AST on the basis of the data structure represented by said abstract syntax, said AST representing the data structure of said abstract syntax; an AST storage, connected to said AST generating unit, for storing said AST therein; and a PDU encoding unit, connected to said AST storage and said VTL generating unit, for encoding said abstract syntax into said PDU on the basis of said AST and said VTL.