An on-board network system includes: communication lines; a plurality of nodes, each of the nodes being directly and physically connected to at least one of the other nodes by the communication lines, the nodes including loop nodes physically connected to a loop by the communication lines, the nodes constituting a first logical network in which the nodes are connected with each other via one route through the communication lines; a communication disruption detecting unit configured to detect disruption of communications via the communication lines; a network reconstruction unit configured to reconstruct a second logical network when the disruption of communications is detected by the communication disruption detecting unit, the second logical network not including a communication line that causes the disruption; and a delivery unit configured to deliver the information of the second logical network to the nodes.