Data during normal driving is generated using travel data during the occurrence of a failure or the like, the travel data being accumulated into ordinary vehicles traveling in a city or other places on a daily basis. A failure diagnosis of a vehicle is performed by comparing a reference value with the time sequence electronic control unit (ECU) data of a plurality of driving parameters during the occurrence of a failure, the data being stored into a storage device in the ECU of the vehicle when a failure of the vehicle has occurred. The time sequence ECU data obtained from a large number of vehicles are sequentially accumulated and stored and the numerical vectors of the accumulated time sequence ECU data are generated. The numerical vectors are clustered and classified into a plurality of clusters according to the characteristics. In each of the plurality of clusters, a range of high occurrence rate values is obtained for the values of the respective driving parameters. The obtained range of high occurrence rate values is then stored as a range of the normal values of the driving parameters, and the range of the normal values is set as the reference value for a failure diagnosis.