A key management method serves as an electronic control unit (ECU) in an onboard network system having a plurality of ECUs that perform communication by frames via a network. The method includes storing, in a first-type ECU, a shared key to be mutually shared with second-type ECUs, and executing encryption processing regarding a framed transmitted or received via the network, based on the shared key. The method further includes executing, by the first-type ECU, inspection of a security state of the shared key stored by the second type ECUs in a case where a vehicle is in at least one of the following particular states, including immediately after the vehicle is not driving and is entering the accessory-on state, immediately after the vehicle is not driving and the vehicle is entering the accessory-off state, and immediately after the vehicle engine is started.