PURPOSE: To check the normality of operation of a monitor, by inputtting a loop back instruction from an input section to a centralized information processing section, generating an alarm signal from an output port via a monitor information processing section of the monitor, and executing remote control from the centralized monitor.

CONSTITUTION: A loop back control instruction addressed to corresponding monitors 2∼8 is inputted from the input section 15 to the centralized information processing section 13. The section 13 interpretes the said instruction and transmits the instruction to the corresponding monitors 2∼8. The addressed monitors 2∼8 interprete reception data at a monitor information processing section 9 and gives an output request to the output port 10. A virtual alarm signal outputted from the output port 10 is detected at an alarm detection section 11 and monitor information is transmitted with processing. That is, loop back is done. The centralized monitor 1 prints and displays required failure status at an output section 14 with monitor information transmitted from the said monitors 2∼8.