A network master (M) having charge of an operation of a main device (L) and one or more network slaves (S1 to S3) each having charge of a safety determining device (A to C) for determining safety which is an operating permission condition of the main device (L) are connected on the network. Each network slave comprises means for selectively performing a first operation for transmitting information including data for maintenance of the safety determining device or both determination output of the safety determining device and data for maintenance of the safety determining device, to the network master and a second operation for transmitting information including the determination output of the safety determining device but not including the data for maintenance of the safety determining device at all or hardly including it to the network master. The first operation in the network slave is performed when the main device is in a stopped state and the second operation in the network slave is performed when the main device is in an operating state.