A communications system includes a transmission device configured to transmit information as a communication message to a network at a predetermined frequency according to a type of information and a reception device configured to receive the transmitted communication message, wherein a first type of information to be transmitted at a first frequency and a second type of information to be transmitted at a second frequency higher than the first frequency are transmitted as different communication messages to the network, and wherein the reception device detects the occurrence of an abnormal state in the network on the basis of the number of receptions of the second type of information until the next first type of information is received after the first type of information is received.