PURPOSE: To enable to readily discover the cause and the position of a troubled part by providing a monitor memory for storing an information monitor for monitoring information to be inputted to a central station and predetermined period information and sequentially updating the information.

CONSTITUTION: A control command of an instructing unit 6 is transmitted through an input interface 13, a transmission buffer memory 16, a transmission controller 20 and a transmission line 5a to a terminal station 4. The station 4 outputs a command corresponding to the transmitted control command to control units 7W9, and transmits the operating information of the units 7W9 to a central station 10. The memory 16 stores the inputted information for the prescribed period. When an information monitor 17 detects the malfunction of the content of the memory 16, it transfers the content of the memory 16 to a monitoring memory 18, and outputs through an output interface 15 to a display unit 11 and a monitor output unit 12.