To provide a bearing unit capable of reducing the weight of the bearing unit, and suppressing generation of abnormal noise while maintaining the constant rigidity of the entire flange.

The bearing unit comprises bearing rings (an outer ring 2 and an inner ring 4) which are arranged facing each other in a relatively rotatable manner, and a plurality of rolling elements 6, 8 which are assembled between the bearing rings in a rolling manner. One bearing ring is fixed to a vehicle body side and maintained constantly in a non-rotatable manner while the other bearing ring is connected to the wheel side and rotated together with the wheel. A flange 12a with wheel side components (such as a disk wheel and a brake rotor) constituting the configuration on the wheel side being fixed thereto is protruded on the other bearing ring. The flange comprises a padded portion P1 having the relatively increased thickness and a lightened portion P2 which is more thin-walled than the padded portion. The thickness of the padded portion is set in a range of substantially 8-13 mm, and the thickness of the lightened portion is set to be in a range of substantially 4-8 mm.

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