An in-vehicle network system includes a plurality of relay devices that is connected to control devices of a vehicle, and communicates with each other and is configured such that each of a plurality of request devices that transmits a response request to the control device is connected to any one of the relay devices, a storage unit that stores a priority of the request devices, a master setter that sets, as the master, the relay device to which the request device having the highest priority has been connected among the relay devices to which any one of the request devices has been connected, and an arbitrator that performs arbitration to prioritize a response request from the request device having the highest priority when the response requests are transmitted from at least two of the request devices when the relay device in which the arbitrator is included is the master.