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 a shared key, acquiring a session key, and executing encryption processing using the session key. The method further includes executing inspection of a security state of the shared key stored in a case where a vehicle is in at least one of the following particular states: the vehicle is not driving and is an accessory-on state; a fuel cap of the vehicle is open, and the vehicle is not driving and is fueling; the vehicle is parked, which is indicated by the gearshift; the vehicle is in a stopped state before driving, which is indicated by the gearshift; and a charging plug is connected to the vehicle, and the vehicle is electrically charging.