A slave station 30 includes a cyclic-memory managing unit 36 that generates data expansion state information indicating whether received cyclic data is expanded into a cyclic memory 33 and data processing state information indicating whether the slave station 30 is in a state in which the slave station 30 can receive data frame at the time of receiving the data frame, and a frame transmitting unit 38 that transmits a state notification frame including the data expansion state information and the data processing state information to a master station. The master station includes a system-abnormal-state determining unit that determines, when a token frame is received, an error state of the slave station 30 using the data expansion state information and the data processing state information of the state notification frame received by a frame receiving unit.