This control device comprises: a communication unit for communicating, via an in-vehicle communication line, with a vehicle-mounted control device; and a control unit for controlling the communicationunit. The control unit executes: acquisition processing for acquiring a first time and a second time, described below; and determination processing for determining, on the basis of the results of comparison between the first time and the second time that have been acquired, the necessity of a rollback to a pre-update control program in the vehicle-mounted control device in which a control programupdate is in progress. First time: the time from the present time, at which an update of the control program is in progress, until the update is completed. Second time: the time from the present timeuntil a rollback to the pre-update control program is completed.