To provide a structure for securing strength and sufficient durability of a bearing unit for a wheel support while suppressing weight increase due to increase of the dimensions or thicknesses of an outer ring and a hub.

The outer ring 3a and the hub 2a are arranged concentrically each other, and in addition, a plurality of balls 4a and 4b are respectively arranged between a plurality of lines of outer ring tracks 14a and 14b arranged in the inner peripheral surface of the outer ring 3a and a plurality of lines of inner ring tracks 8a and 8b arranged in the outer peripheral surface of the hub 2a rotatably. The ball diameters of the balls 4a arranged in a line on the axial outside are larger than those of the balls 4b arranged in a line on the width direction inside. The pitch circle diameters of the balls 4a arranged on the axial outside are set to be same as pitch circle diameters of the balls 4b arranged in a line on the axial inside.