The invention finely controls traveling of a vehicle and has small electric vehicle with electric aid function to improve in serviceability for a user such as failure diagnosis and determination of a failure position by communicating information between a battery side controller and a vehicle side controller. It is equipped with a battery management controller 100c and a vehicle controller 101a, is connected to a mode switch means 102 and an information display means 103. When the failure diagnosis mode is set, the display controller displays the failure diagnostic information on the information display means 103. |