Provided is an update management method that causes an external tool, capable of transmitting an update message to update data such as shared keys and the like within electronic control units (ECUs) making up an onboard network, to update shared keys and the like within the ECUs, while reducing the risk of all ECUs being unauthorizedly rewritten in a case where secret information given to the external tool is leaked. The update management method is to receive and verify update authority information indicating authority of the external tool, and in a case that an update message instructing updating of shared keys or the like of one or multiple ECUs has been transmitted from the external tool (step S1010), if the verification is successful and the update authority information indicates that the transmission of the update message is within the range of authority of the external tool (step S1013), the update is executed at the ECU (step S1019), and otherwise, update at the ECU is inhibited.