To reduce adverse effects on a magnetic encoder or a sensor and secure sensor's stable detection of output, thereby improving reliability.

A sensor holding body includes a cylinder portion that is fitted to an outer ring and a circular flange portion that is extended from an edge part of the cylinder portion, covers a magnetic encoder portion, and faces thereto. In the circumferential direction Y of the back surface of the circular flange portion, a contact portion 12d to be struck to an edge surface of the outer ring is formed and is struck to the edge surface of the outer ring. In the condition that the cylinder portion is fitted to and loaded with the outer ring, at a lower portion in the radial direction X of the sensor holding body, a drain port 12c is formed to communicate with outside an internal space between the circular flange portion and the edge surface of the outer ring. The contact portion 12d is not formed in the region of arc which is between a predetermined length from both edge portions of the drain port 12c in the circumferential direction Y outside.

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