PURPOSE: To ensure the monitor control of a monitored control station by providing plural signal detection switch sections, a logic section and a network section to a monitor control station and the monitored control station and preventing collision or bypass of an inputted signal.

CONSTITUTION: A detector 34 detects a level of a carrier and sends a flag whose level is '1' representing a signal input to a logic section 51 when the detected level is a prescribed level or over. When a selector selects a flag of the detector 34 and detects '1', the logic section 51 stops the selection and sends a switch control signal to a switch 31. Then an analog signal fed to the switch is distributed by a distribution section 61 and sent to a transmission line from an output port. In this case, even when an analog signal is inputted via other input port, the selection of the logic section is stopped and no switch control signal is fed to the switch connecting to the other port.