Abstract: In the present invention, a gap 73 is formed between an axial-direction outer end portion of an internal gear 30 and an attachment ring 70, and the gap 73 is filled with a lubricant 74. This configuration makes the rotation of the internal gear 30 relative to the attachment ring 70 smoother and also can reduce the generation of abnormal sound. Further, the thickness of second lock plates 60B, 60D between which and an inner surface of a guide bracket 20 rolling balls 65 are not interposed is equal to or more than the distance between the inner surface of the guide bracket 20 and outer surfaces of first lock plates 60A, 60C, between which the rolling balls 65 are interposed, and preferably substantially equal to the distance. This reduces the precession of the first lock plates 60A, 60C and makes it possible for a load input in an axial direction to be mainly received by the second lock plates 60B, 60D.