An electronic control unit improves a serviceability for a user of a vehicle by performing an abnormality diagnosis of the vehicle based on (i) storing a readiness information indicating that readiness is complete in a readiness storage, (ii) storing a history of normal diagnosis results from an abnormality diagnosis in a history storage, and (iii) relaxing a readiness complete condition for storing the readiness information in the readiness storage under control of a microcomputer, when the history stored in the history storage includes information of existence of the normal history. By using the relaxed condition for the abnormality diagnosis, readiness information is more quickly stored when performing the abnormality diagnosis. Also, diagnostic process is completed more quickly, facilitating the repair process and enabling the user to more quickly receive a desired service.