Each of a control panel node 2 and an input/output node 3 has a safety data processing unit 7 and a high reliability communication unit 8. At the time of transmission, the safety data processing unit 7 creates a safety data packet including data about safety and the high reliability communication unit 8 creates a communication data packet including the safety data packet from the safety data processing unit 7 and transmits the communication data packet a predetermined number of times. At the time of reception, the high reliability communication unit 8 acquires and outputs a safety data packet by determining one of one or more communication data packets having the same contents which it has received properly as an effective packet, and the safety data processing unit 7 analyzes a state concerning the safety of a system on the basis of the data about safety acquired from the safety data packet from the high reliability communication unit 8.