A communication system that comprises a ring type network, wherein in the ring type network a plurality of network nodes (14a to 14d) forms a ring-like shape, and the ring type network is adapted to perform communications in one direction; wherein each of the network nodes (14a to 14d) comprises: a data transmission unit (33a to 33d) having a data transmission block (22a to 22d) adapted to transmit, as transmission data, data generated by a host network node of the network nodes (14a to 14d), and a data relay block (20a to 20d) adapted to relay, as relay data, transmission data transmitted from other network nodes of the network nodes (14a to 14d); a data reception unit (24a to 24d) adapted to receive, as reception data, the transmission data transmitted from the other network nodes; and an output switching unit (26a to 26d) adapted to switch and output as output data either one of the data from among the transmission data transmitted by the data transmission block (22a to 22d), or the relay data relayed by the data relay block (20a to 20d), characterized in that at least one from among the data transmission unit (33a to 33d) and the output switching unit (26a to 26d) of each of the network nodes (14a to 14d) includes an error detecting unit (34a to 34d, 90a to 90d) adapted to detect an abnormality in the data while outputting the data in accordance with an on-the-fly method; and when the error detecting unit (34a to 34d, 90a to 90d) detects an abnormality in the data, the data transmission unit (33a to 33d) or the output switching unit (26a to 26d) that includes the error detecting unit (34a to 34d, 90a to 90d) truncates output of the data midway, adds error data (44a to 44d) to the end of the truncated data, and then outputs the data, wherein the error data is indicative of the fact that an abnormality exists in the data. a data transmission unit (33a to 33d) having a data transmission block (22a to 22d) adapted to transmit, as transmission data, data generated by a host network node of the network nodes (14a to 14d), and a data relay block (20a to 20d) adapted to relay, as relay data, transmission data transmitted from other network nodes of the network nodes (14a to 14d); a data reception unit (24a to 24d) adapted to receive, as reception data, the transmission data transmitted from the other network nodes; and an output switching unit (26a to 26d) adapted to switch and output as output data either one of the data from among the transmission data transmitted by the data transmission block (22a to 22d), or the relay data relayed by the data relay block (20a to 20d), characterized in that at least one from among the data transmission unit (33a to 33d) and the output switching unit (26a to 26d) of each of the network nodes (14a to 14d) includes an error detecting unit (34a to 34d, 90a to 90d) adapted to detect an abnormality in the data while outputting the data in accordance with an on-the-fly method; and when the error detecting unit (34a to 34d, 90a to 90d) detects an abnormality in the data, the data transmission unit (33a to 33d) or the output switching unit (26a to 26d) that includes the error detecting unit (34a to 34d, 90a to 90d) truncates output of the data midway, adds error data (44a to 44d) to the end of the truncated data, and then outputs the data, wherein the error data is indicative of the fact that an abnormality exists in the data.