An electronic control unit continuously stores data indicating the monitoring frequency of the diagnosis items determined by the RateBase monitoring method and is increased one by one with the maximum amount of change in the single operating period to an EEPROM. When any data is incremented by one, this data is entered to the EEPROM and the write completion flag for this data is turned on. Thereafter, in this operating period, this data is never entered to the EEPROM. Therefore, reduction and increase in the number of times of data entry to the programmable non-volatile memory and reliable storage of this data can be realized effectively.