A seal ring has an annular body which is constituted so that a disc section extended to a radially inward direction is overlapped with one end of a cylindrical section, and a sensor member for detecting rotation of an inner ring is provided to an outer periphery of the cylindrical section, and an outer case for covering the sensor member is provided by insert-forming. The outer case contracts after it is formed and is press-contact with the cylindrical section so that a gap is hardly generated.