An in-vehicle network system includes a plurality of controllers; a plurality of communication lines connected to the controllers, respectively, a relay device connected to the controllers via the communication lines, and a retention determiner configured to determine whether retention of data occurs in the relay device. The relay device is configured to relay data between the communication lines, and configured to preferentially relay data having a priority equal to or higher than a predetermined priority such that the data reaches the controller that is a transmission destination among the controllers within a first predetermined time when the retention determiner determines that the retention occurs.