1. A method for making a failure diagnosis by comparing driving data relating to a plurality of parameters stored in an electronic control unit of a vehicle when a failure occurs in the vehicle with reference values as reference driving data during normal time, the method comprising: selecting, via a failure diagnostic unit, reference value groups categorized by driving environment, which are entirely approximated to inspection data, by approximate search between the inspection data which is the driving data stored in the electronic control unit of the vehicle to be subjected to failure diagnosis and the reference value groups categorized by the driving environment whose normal value ranges are set for the plurality of parameters categorized by the driving environment; and identifying one of the plurality of parameters having a substantially large deviation from the normal value range by comparing the inspection data with the normal value ranges for the plurality of parameters in the selected reference value groups categorized by the driving environment, the failure diagnostic unit searching a failure cause based on the identified one of the plurality of parameters.