PURPOSE: To reduce axial dimension, to improve degree of sealing between a rotary member and fixed member, and to facilitate installation.

CONSTITUTION: A cylindrical part 3 is fitted to a shaft 1, an encoder formed of elastomer loaded with magnetic particles is fitted to a rotary disc 2 integrated with the cylindrical part 3, a cylindrical part 7 is fitted to a fixed member 6, a sealing member 10 is fitted to a fixed disc 8 integrated with the cylindrical part 7, and the sealing member 10 is provided with an axial tongue-like piece 11a abutting on the free surface of the rotary disc 2 and a radial tongue-like piece 11b abutting on the outside surface of the cylindrical part 3. Further, an annular member 12 is provided on the end of the sealing member 10, the encoder 4 is provided with a tongue-like piece 13 abutting on the shaft 1, and the outer side surface of the encoder 4 and the plane (p) of a lateral side 61 are substantially in one plane.