To measure the unevenness on the surface of a commutator with a motor installed in a vehicle without dismounting or without lifting the vehicle with a jack.

This eccentricity measuring device for measuring the peripheral surface of the commutator of the motor is equipped with (i) a fixing member being fixed to a brush holder of the motor; (ii) a noncontact displacement detector installed in the fixing member and measuring the displacement of the commutator; and (iii) a data processing device installed in the displacement detector.