This invention enables an abnormality analysis to be easily and reliably performed in the FA system of the EtherCAT (registered trademark). A controller has a protocol monitor function of operating in a monitor system program, and constantly monitors data communicated with a remote device. The controller has an abnormality diagnosis function of detecting abnormality, and thus holds the data monitored immediately before when abnormality is detected. As the protocol monitor function is incorporated, a protocol monitor does not need to be newly plugged into the network as an external device after the occurrence of abnormality, and the data that becomes the cause can be held from the abnormality that occurred first by monitoring from the beginning of the operation of the system and can be used for analysis.