Embodiments of the invention provide, in a compact magnetic disk drive, a stable structure capable of reducing self-oscillation occurring from disk spinning, while suppressing generation of dust and dirt. In one embodiment, a magnetic disk is mounted on a hub of a spindle motor. A clamp is further attached to secure the magnetic disk in position. A plurality of opening portions are formed in the clamp around an axis. A counterweight formed of an elastic material is mounted in the opening portion by being press-fitted therein. The counterweight is less likely to generate dust and dirt through friction during insertion and looseness. A gap is provided between the clamp and the hub at locations near the opening portions. The counterweight has a protruded portion on a side surface of a circular column portion. The protruded portion slides into the gap between the clamp and the hub, counteracting a tendency of the counterweight to drop out of position.