A microcomputer and method are provided capable of restarting a rewrite program without the need for changing a mode using an external terminal when rewriting nonvolatile memory fails. A CPU of a microcomputer executes a rewrite program to clear FLASH status 0 of flash memory and rewrite all areas in it. The CPU finally writes a rewrite completion code to FLASH status 0. The CPU executes a determination program to read FLASH status 0 of the flash memory. The CPU reads ID status information when read data does not match the rewrite completion code. The CPU re-executes the rewrite program when the data matches ID status information.