An electronic control unit installed in a vehicle performs diagnosis of plural diagnosis objects based on sensor signals, and stores diagnosis results that indicate abnormality of the diagnosis objects in a DTC storage area only when the DTC storage area does not store data having an initial value. The data in the DTC storage area is, in other words, rewritten to different values from the initial value for allowing storage of DTCs when the electronic control unit determines that all of the diagnosis objects are in a normal condition with no indication of abnormality after performing diagnosis, thereby making it possible to exclusively prevent useless abnormality information detected during a vehicle manufacturing process from being stored in an rewritable non-volatile memory.