To secure safe and accurate program update even when any error is generated during program update in a device where a program for shifting to a rewrite mode is recorded so as to be rewritable.

This information processor is configured to judge whether or not rewrite conditions are satisfied from an application (rewrite condition decision) program stored in a flash ROM so as to be rewritable, and to judge that rewrite conditions are satisfied when a vehicle status or peripheral environment is put in a predetermined status, and to perform a rewrite program, and to update an application program in an ROM. Also, the sum value of a rewrite condition decision program is calculated (S130), and compared with a specific value, and whether or not the write condition decision program is correct is judged before the performation of the rewrite condition decision program (S140), and when it is judged that the rewrite condition decision program is not correct, a warning light 3 is turned on (S150) under the control of a boot program.