PURPOSE: To minimize the change of communication protocol processing at the time of changing an application layer protocol by preparing one conversion information table and two processing parts and executing encoding/decoding processing.

CONSTITUTION: In the case of executing encoding processing, a conversion control part receives an internal expression table area and a table A from an encoding processing requesting part, determines a processing-sorted transfer syntax A and finds out P record ID = 01 corresponding to a transfer syntax A in a protocol data conversion information table. In the case of executing decoding processing, the conversion control part receives the transfer syntax, the internal expression table area and the table A from a decoding processing requesting source, points out the head of the transfer syntax and finds out leading P record ID = 01 in the protocol data conversion information table.