Each of a master node and a slave node has a safety data processing unit and a high reliability communication unit. At the time of transmission, the safety data processing unit creates a safety data packet including data about safety and the high reliability communication unit creates a communication data packet including the safety data packet and transmits the communication data packet a predetermined number of times. At the time of reception, the high reliability communication unit 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 analyzes a state concerning the safety of a system on the basis of the data about safety acquired from the safety data packet.