PROGRAM UPDATE METHOD AND UPDATE SYSTEM

An update system includes: a first server that stores a control program; a second server that stores a common program; a difference extraction device that generates difference data between the common program and the control program; and a reprograming tool that transmits the difference data to a vehicle equipped with an ECU to be updated. The difference extraction device searches a search range by a search unit to find whether search target data of the control program is included in the common program, and generates the difference data. The search range includes, for example, an address of the search target data in the control program and addresses rearward of that address. An offset area is provided in a head area of the common program.