An update management method, used in an onboard network system having a plurality of electronic control units (ECUs) that perform communication via a bus, to which an external tool is connected, the method comprising: storing, by a master electronic control unit (master ECU) which is included in the ECUs, a shared key used in transmission of a first session key for an encryption processing between the master ECU and an electronic control unit which is other than the master ECU, and an expiration date of the shared key; in a case where the master ECU receives an update message from the external tool instructing updating of the shared key, verifying update authority information indicating authority of the external tool, and determining whether or not a transmission of the update message is within a range of an authority of the external tool; (i) in a case where verification of the update authority information is successful, and the transmission of the update message by the external tool is determined to be within the range of the authority of the external tool, transferring the update message to the bus; (ii) in a case where the verification of the update authority information fails, or the transmission of the update message by the external tool is determined to be outside of the range of authority of the external tool, not transferring the update message to the bus; acquiring external point-in-time information; determining whether or not the external point-in-time information is before the expiration date; and in a case where the external point-in-time information is a predetermined amount of time before the expiration date or the expiration date has passed, transmitting an alert message prompting to update the shared key. storing, by a master electronic control unit (master ECU) which is included in the ECUs, a shared key used in transmission of a first session key for an encryption processing between the master ECU and an electronic control unit which is other than the master ECU, and an expiration date of the shared key; in a case where the master ECU receives an update message from the external tool instructing updating of the shared key, verifying update authority information indicating authority of the external tool, and determining whether or not a transmission of the update message is within a range of an authority of the external tool; (i) in a case where verification of the update authority information is successful, and the transmission of the update message by the external tool is determined to be within the range of the authority of the external tool, transferring the update message to the bus; (ii) in a case where the verification of the update authority information fails, or the transmission of the update message by the external tool is determined to be outside of the range of authority of the external tool, not transferring the update message to the bus; (i) in a case where verification of the update authority information is successful, and the transmission of the update message by the external tool is determined to be within the range of the authority of the external tool, transferring the update message to the bus; (ii) in a case where the verification of the update authority information fails, or the transmission of the update message by the external tool is determined to be outside of the range of authority of the external tool, not transferring the update message to the bus; acquiring external point-in-time information; determining whether or not the external point-in-time information is before the expiration date; and in a case where the external point-in-time information is a predetermined amount of time before the expiration date or the expiration date has passed, transmitting an alert message prompting to update the shared key.