Key management method, vehicle-mounted network system, and key management device

Provided is a key management method to secure security in an onboard network system having multiple electronic control units storing a shared key. In the key management method of the onboard network system including multiple electronic units (ECUs) that perform communication by frames via a bus, a master ECU (400) stores a shared key to be mutually shared with one or more ECUs (100a through 100d). Each of the ECUs (100a through 100d) acquire a session key by communication with the master ECU (400) based on the stored shared key, and after this acquisition, executes encryption processing regarding a frame transmitted or received via the bus, using this session key. In a case where a vehicle in which the onboard network system is installed is in a particular state, the master ECU (400) executes inspection (e.g., steps S1201, S1203) of a security state of the shared key stored by the ECU (100a) or the like.