In a diagnostic device and a diagnostic method for a motor unit of an electric vehicle, an oil degradation/others detection unit is provided which is configured to detect at least any one detection item of a contamination degree, a degradation degree, and an oil amount of an oil used for cooling a motor unit or a reduction gear unit during a non-traveling period when the vehicle is powered on. An abnormalities-time control unit is provided which is configured to send notice of abnormalities of an oil supply system or not permit start of rotation of the motor unit when a detection value detected by the oil degradation/others detection unit is out of a setting range.