An on-board network system is presented. The on-board network system sends a sleep-entered message to a communication bus. The sleep-entered message is sent under a condition that a sleep condition is satisfied on a basis that a network management (NM) message is ceased during state transition process in which node's state transfers from a normal state to a power-saving state. A monitoring ECU corresponding to a master performs an abnormality detection process. In the abnormality detection process, the monitoring ECU detects an abnormality state of the state transition process based on whether or not the sleep-entered message is sent from any one of nodes, thereby it is possible to detect the abnormality state not only during each node is a normal state but also during a bus-sleep state.