A spindle structure in which a noise occurrence due to a tap hole and a screw is effectively prevented. A nut for adjusting a preload of a bearing is threadedly engaged with an outer circumference of a spindle. On an edge of the nut, a plurality of tap holes for balance adjustment are provided at even intervals on a circumference centered around the axis of the spindle and to be parallel to the axial direction of the spindle. These tap holes are countersunk, and a plate screw is threadedly engaged so that an edge of each plate screw is flush with a surface of the nut. In addition, the plate screw is used as a balance weight by changing the length thereof.