In order to detect and prevent attacks in which the network is infested with unauthorized data to cause malfunction, the invention aims, on the basis of network cycle information, to detect errors and to prevent unauthorized data forwarding in a network system such as a vehicle-mounted network. This network device is provided with a communication unit which receives data, a time management unit which manages the reception time at which data is received, and a control unit which processes data. If, for received data used as a reference, first data is received having the same identifier and a reception interval shorter than a prescribed cycle, the control unit waits to receive data having the same identifier as the first data from the reception time of the received data used as a reference up until the prescribed cycle has elapsed, and if during this interval second data is received having the same identifier as the first data, the control unit determines that an error has occurred and performs cycle abnormality detection processing (see Fig. 1 ).