1. An electronic control unit comprising: a memory area in a non-volatile memory for storing a control program controlling a predetermined control object, the memory area being rewritable, and having a plurality of sub-memory areas exclusively defined in a non-overlapping manner; and an operation unit performing a process according to the control program stored in the non-volatile memory and performing a rewrite process to rewrite contents of the control program stored in the memory area when a predetermined rewrite condition is met, wherein from among the plurality of sub-memory areas of the memory area, a read sub-memory area, from which the control program is read by the operation unit to perform the process according to the control program, is different and separate from a rewrite sub-memory area, which stores program contents of the control program that is rewritten by the rewrite process.