An electronic control device for a vehicle configured to be able to rewrite a program related to vehicle control by connecting an external device with the vehicle and by accessing an in-vehicle LAN includes a portable authentication terminal separated from the external device; an in-vehicle authentication system connected with the in-vehicle LAN, configured to authenticate the portable authentication terminal and to be able to transmit a authentication result to the in-vehicle LAN; a repeater configured to relay communications between the external device and the in-vehicle LAN; and a communication authorization unit connected to the in-vehicle LAN and configured to authorize the communications between the external device and the in-vehicle LAN through the repeater if receiving the authentication result of a successful authentication of the portable authentication terminal by the in-vehicle authentication system.