The disc-driving unit is formed of a rotor having a turntable on which a disc is mounted, a stator having a bearing housing, and a drop-guard mechanism for protecting the rotor from coming out of the stator. On the turntable, a plurality of engaging sections are integrally formed, while on the bearing housing, an engaged section is integrally formed. The engaged section has a plurality of recesses each of which has an opening width wide enough for passing through each of the engaging sections and a plurality of guides for helically guiding the engaging sections into the recesses. A helical engagement of the engaging sections and the engaged section completes the drop-guard mechanism.