To provide an auto-balancer, using balls which are able to exhibit functions as an auto-balancer by using a simple structure consisting of a device added to the shape of a case bottom without sacrifing cost, and exhibit the function as the auto-balancer without causing a problem of inertia resulting in delayed start-up.

This spindle motor involves an auto-balancer, in which a plurality of balls are disposed at a shallow case 1 integrally-formed with a rotating shaft S, wherein a small taper 1a is disposed at the case 1 so that the balls 3 are gathered in the center at least in a resting condition, and a ball housing 1a formed, so that the balls 3 jump out against the centrifugal force when the rotational speed of the balls 3 reaches a prescribed one is disposed near the rotating shaft. A ball guide 4 is mounted on the bottom of the case 1, and the ball 3 is formed out of non-magnetic material.