There is disclosed a bearing unit (5, 5a) for a wheel comprising a stationary race (6) having stationary race track surfaces (11a, 11b) on a peripheral surface on the stationary side thereof to be supported and fixed by a suspension system in its used state, a rotating race (8) having rotating race track surfaces (14a, 14b) on a peripheral surface on the rotating side so as to fix a wheel and a braking rotating member in its used state, and a plurality of balls (17) provided between the rotating race tracks and the stationary race tracks, wherein a standard value for the mutual difference in diameter between these balls is not more than 1.0 .mu.m.