A vehicle failure is diagnosed based on data recorded in an electronic control unit (ECU) of a vehicle. Driving data pertaining to multiple parameters which are stored in the electronic control unit of the vehicle for when a vehicle failure occurs are compared with reference values representing data during normal driving in order to diagnose a failure. The device utilizes an approximate search between inspection data, that is, driving data which are stored in the electronic control unit of the vehicle to be diagnosed for failure, and reference value groups categorized by driving environment, wherein a normal value range is set for each of the aforementioned parameters categorized by driving environment, and selects the reference value group categorized by driving environment which approximates the inspection data. Furthermore, the device compares the inspection data with the normal value ranges of each parameter of the selected reference value group categorized by driving environment so as to identify parameters which deviate significantly from the normal value ranges and find the cause of the failure based on the parameters identified.