A PLC communication system comprising: a programmable logic controller (PLC, 20); a plurality of processing devices (150, 250, 251 through 258); at least three communication processing nodes (41, 140, 240, 241 through 248) provided at the PLC and at the plurality of processing devices respectively to establish communication among the PLC and the plurality of processing devices; and at least two interconnecting devices (HUBs) and a plurality of communication lines (120, 220,121 through 139, 221 through 236) for connecting the communication processing nodes to form a communication route among the communication processing nodes, characterized in that each communication processing node (41, 140, 240, 241 through 248) is connected to a respective interconnecting device via one communication line (120, 220,121 through 139, 221 through 236); the communication processing nodes (41, 140, 240, 241 through 248) are configured to send and receive a predetermined token among each other in a predetermined order via the communication lines (120, 220,121 through 139, 221 through 236), wherein, if a first communication processing node does not receive the predetermined token within a predetermined time from a second communication processing node via one communication line connected to the first communication processing node, the first communication processing node is configured to reissue the predetermined token and to forward the reissued predetermined token to a third communication processing node via the one communication line connected to the first communication processing node, the programmable logic controller is configured to detect a communication failure state in a communication route formed between the first communication processing node and the second communication processing node when the first communication processing node reissues the predetermined token via the one communication line connected to the first communication processing node, and to count the number of occurrences of the detected communication failure state, and the programmable logic controller is configured to determine a portion in the communication route to be abnormal based on the number of occurrences of the detected communication failure state in the interconnecting devices and the plurality of communication lines, the PLC communication system further comprising a display device configured to display the abnormal portion in the communication route. a programmable logic controller (PLC, 20); a plurality of processing devices (150, 250, 251 through 258); at least three communication processing nodes (41, 140, 240, 241 through 248) provided at the PLC and at the plurality of processing devices respectively to establish communication among the PLC and the plurality of processing devices; and at least two interconnecting devices (HUBs) and a plurality of communication lines (120, 220,121 through 139, 221 through 236) for connecting the communication processing nodes to form a communication route among the communication processing nodes, characterized in that each communication processing node (41, 140, 240, 241 through 248) is connected to a respective interconnecting device via one communication line (120, 220,121 through 139, 221 through 236); the communication processing nodes (41, 140, 240, 241 through 248) are configured to send and receive a predetermined token among each other in a predetermined order via the communication lines (120, 220,121 through 139, 221 through 236), wherein, if a first communication processing node does not receive the predetermined token within a predetermined time from a second communication processing node via one communication line connected to the first communication processing node, the first communication processing node is configured to reissue the predetermined token and to forward the reissued predetermined token to a third communication processing node via the one communication line connected to the first communication processing node, the programmable logic controller is configured to detect a communication failure state in a communication route formed between the first communication processing node and the second communication processing node when the first communication processing node reissues the predetermined token via the one communication line connected to the first communication processing node, and to count the number of occurrences of the detected communication failure state, and the programmable logic controller is configured to determine a portion in the communication route to be abnormal based on the number of occurrences of the detected communication failure state in the interconnecting devices and the plurality of communication lines, the PLC communication system further comprising a display device configured to display the abnormal portion in the communication route.