Communication system and data rewriting method which can reduce the cost of an ECU are provided. A communication system includes a master ECU and a slave ECU configured to receive a communication frame from the master ECU, wherein the slave ECU includes a region for an operating program that controls a load connected to this slave ECU, and a region for a set value. The slave ECU is configured to rewrite the set value based on a communication frame transmitted from the master ECU.