A fuel-injector-replacement device obtains a learning value of characteristic data of an injector while an engine is running. The learning value stored in an ECU is updated into the learning value. When the engine is stopped, the characteristic data stored in an INJ-EEPROM are updated into the characteristic data stored in the ECU. When the engine is started, it is determined whether the characteristic data stored in the ECU agrees with the characteristic data stored in the INJ-EEPROM. When these data do not agree with each other, it is determined that the fuel-injector is improperly replaced without updating the characteristic data stored in the ECU.