To provide a device and a method for diagnosing an electric vehicle driving motor in which oil deterioration or the like can be easily measured, a decelerator failure can be prevented or a cooling effect of the motor can be kept high, and vehicle maintenance can be promoted.

The diagnostic apparatus includes a detection means 37 for oil deterioration or the like for detecting at least one of detection items of an oil contamination level, a deterioration level, and an oil amount when the power of the vehicle is already turned on but the vehicle is not driven. The apparatus includes an abnormality detection-time control means 40 for outputting an abnormality with an oil supply system or preventing the rotation of the motor 6 from being started when a detection value detected by the detection means 37 for oil deterioration or the like does not fall within a set range.