ELECTRONIC CONTROL UNIT AND ELECTRONIC CONTROL SYSTEM

Provided is a program updating process according to which even when power supply to an electronic control unit stops during program updating, invalid data's remaining in a storage area at completion of program updating is prevented without acquiring a power supply status of a vehicle. An electronic control unit includes a communication interface, a nonvolatile memory, and an arithmetic processing unit that executes a program. The nonvolatile memory includes a program storage area and a data storage area. The program storage area has a program updating process unit that carries out an updating process, and a program updating target unit that stores an updating program. The arithmetic processing unit reads the program updating process unit as the program and executes the program, divides an updating program received by the communication interface into divided updating programs each having a given size, and writes each of the divided updating programs to the program updating target unit. The arithmetic processing unit stores updating progress data in the data storage area, the updating progress data indicating a position of the divided updating program having been written to the program updating target unit.