A proxy processing method to flexibly add/modify customized operations, such as customized user authentication and accounting, is provided. A state transition engine stores state information and moves processing forward by updating states. It has the following characteristics. (1) a state transition table indicating a proxy processing procedure is generated from a state definition file at start-up (i-(i)). (2) A state transition engine implements proxy processing according to the state transition table (1-(2)). (3) A feature is provided to add an extended region for extended features to the session management table. (1(3)). Also, with regard to extended logging: (1) An extended log definition file indicates extended log information to be logged (2-(1)). (2) Extended log information based on the extended log definition is stored in the session management table (2-(2)). (3) Each access log provides two types of separate information, a fixed-length standard log and a variable-length extended log.