PURPOSE: To eliminate data alterations in an actual site by providing a command means which sends a fetch command for classification data to a sensor, a means which registers the classification data sent back from the sensor according to the command, and a receiver.

CONSTITUTION: The classification fetch command is a command indicating the fetch of the classification data from the sensor 3 and repeaters 21-23. At this time, dummy data are written as the data, sent signals are transmitted to the sensor 3 or repeaters 21-23, and the classification data are received from the sensor 3 and repeaters 21-23 in a reply waiting period. For the signal transmission between the receiver 1 and, the sensor 3 and repeaters 21 and 22, the receiver 1 specifies the respective sensors, 3 and repeaters 21 and 22 cyclically in order with address data and sends the sent signals to a transmission line and when the address data of the signal matches their own addresses, the sensor 3 and repeaters 21 and 22 inputs the signal and return sent data after specific processing.