A software update device is provided with an update control unit that that performs a software update using a new software version supplied over a network. If a software update is to be performed for a first electronic control device and a second electronic control device that operates in cooperation with the first electronic control device, the update control unit performs the software update for the first electronic control device such that the first electronic control device after the software update has been performed is capable of selectively executing the operation logic of a first new software version and the operation logic of a first old software version.