PURPOSE: To attain the long service life and reliability of a device in an operation station for periodically executing semi-double poling type signal transmission/reception in an opposed state to a monitor control station and executing semi-double selecting type signal transmission/reception when it is irregularly selected.

CONSTITUTION: A microcomputer 10 analyzes the contents of a file 14 monitoring the rereceiving status of the same command signal for t seconds within 10 seconds after sending a response signal to be edited in a storage device 11, determines a transmission output level and outputs a command to a carrier control part 12. When a carrying part 13 is started based upon a command, the response signal is sent at the set transmission output level, and after the lapse of t seconds, the contents of the file 14 monitored based upon the monitoring result of the same signal rereceiving status.