PURPOSE: To prevent performance from worsening in an internal mechanism of bearing or the like by providing a close contact seal part formed in one of mutual opposed surfaces of a pulley and an outer ring to come into close contact with the other, so as to interrupt infiltrating dust and water into inside a coupling.

CONSTITUTION: Of opposed surfaces to a flange 34a of an outer ring 34 in a flange 32a of a pulley 32, in a position in an internal peripheral side from a bolt or rivet 33, a seal surface 32c is formed to be retracted from a butt surface to the flange 34a of the outer ring 34. On the other hand, in the flange 34a of the outer ring 34, an annular seal protrusion 41 protruded from a peripheral end part of a coupling rubber 37 is integrally formed as a close contact part, and this annular seal protrusion 41, by tightening the flange 34 of the outer ring 34 to the flange 32a of the pulley 32 by the bolt or rivet 33, is brought into close contact with the seal surface 32c by a suitable collapsing margin. Consequently, infiltrating dust and water, from between the pulley 32 and the flanges 32a, 34a of the outer ring 34 to an internal space S having a ball bearing 31, can be surely interrupted.