Provided are a rewriting system capable of shortening rewrite time, a rewriting system and a computer used for the rewriting system. The rewriting system includes an ECU and a rewriting device for at least transmitting to the ECU a rewrite data of memory contents stored in a flash ROM 14B (memory), the flash ROM 14B (memory) includes a rewrite target area A2 in which the memory contents to be rewritten are stored and a non-rewrite target area A1 in which the memory contents not to be rewritten are stored, and the rewriting device transmits the rewrite data of the memory contents to the ECU only for the memory contents of the rewrite target area in the flash ROM 14B (memory).