An in-vehicle network system includes: first processors connected to a bus that is applied to a partial network; and a second processor. Each of the first processors or the second processor send at least one of the first processors in the sleep mode to switch them to the normal mode. Each of the first processors is configured to: assign at least one standby time to each of the first processors and differentiate a standby time assigned to at least one of the first processors from a standby time assigned to other of the first processors, start measuring one of the standby times when sending or receiving one of the NM messages and determine whether or not the standby time has elapsed, and switch at least one of the first processors in which all of the standby times have been determined to have elapsed, to the sleep mode.