In an electronic control system having a CPU, a RAM and an EEPROM, an original data stored in the EEPROM is written into the RAM to be updated in a control calculation processing of the CPU. If the updated data is the type which is to be written back into the EEPROM, a writing request is issued at the same time of writing into the RAM and an address and size of the updated data are also written into the RAM as a related data. Actual rewriting of the updated data into the EEPROM is executed based on the related data in a separate data rewriting processing in response to the writing request. Thus, interruption of the application processing for the data rewriting into the EEPROM can be shortened, and the updated data can be prevented from being broken and lost even upon instantaneous power-off.