ON-BOARD UPDATE DEVICE, UPDATE PROCESSING PROGRAM, PROGRAM UPDATE METHOD, AND ON-BOARD UPDATE SYSTEM

An on-board update device that acquires an update program transmitted from an external server located outside a vehicle, and performs processing to update a program for an on-board ECU provided in the vehicle, the on-board update device including a control unit that controls transmission of the update program to the on-board ECU, wherein, when the transmission is to be resumed after an interruption of the transmission, if identification information of the on-board ECU is different from the identification information before the interruption of the transmission, the control unit determines that the on-board ECU has been replaced during the interruption of the transmission, and performs predetermined processing.