A first relay (30) and a plurality of second relays (40) are interposed between a data collection device (10) and a plurality of monitoring terminals (20). The first relay (30) is connected to the data collection device (10). The plurality of the second relays (40) is connected to the first relay (30). Each of the second relays (40) is connected to a plurality of the monitoring terminals (20). The data collection device (10) and the first relay (30) are communicated with each other according to a main protocol as in the past. The second relay (40) and the monitoring terminal (20) are communicated with each other according to the main protocol. By contrast, the first relay (30) and the second relay (40) are communicated with each other according to an auxiliary protocol different from the main protocol.