A bearing ring for a strain wave gearing includes an inner ring that is rotatable together with an elliptic cam and elastically deformable, an outer ring that is rotatable together with a flex spline and elastically deformable, a plurality of balls provided between the inner ring and the outer ring, and a cage with a plurality of pockets which is arranged in a circumferential direction and in which the balls are housed. The cage has a ring portion and a plurality of cage bars extending from the ring portion in the axial direction. The pockets are each located between the cage bars adjacent to each other in the circumferential direction. A circumferential clearance between the ball and the cage bar is equal to or larger than a radial clearance of an outer annular space between the outer ring deformed into a non-circular shape and the cage.