To simplify a program and to accelerate processing rate by respectively executing functions for converting, distributing and processing messages corresponding to high-order modules through a network module.

When sending a message from a network module 50 on a hierarchy 3 at a fixed station to a subscriber module 12 or a maintenance module 14, the module to send the message is recognized, and the message is converted to a correspondent message format and directly transferred without through a message converting and distributing module. When a message is sent from the subscriber module 12 or the maintenance module 14, on the other hand, the message is analyzed and converted corresponding to the network module 50. Thus, the converting and distributing module is unnecessitated, a memory for message storage with the network module is unnecessitated, any protocol is not required, either, a program amount is reduced and its preparation is facilitated as well.