A fastening member is integrated with a boot body, at least one of a fastening leading end portion or a fastening tail end portion expands or reduces the diameter by elastic deformation, and when the fastening leading end portion or the fastening tail end portion expands the diameter, the fastening member forms the shape of a trumpet. Since the fastening member is integrated with the boot body, a cylindrical small-diameter portion can be readily built in the other part. Since the fastening member forms the shape of the trumpet when the fastening leading end portion or the fastening tail end portion expands the diameter, at least one of the fastening leading end portion and the fastening tail end portion expands or reduces the diameter in the circumferential direction uniformly, and the contact pressure by which the cylindrical small-diameter portion comes into pressure contact with the other part becomes uniform in the circumferential direction, so that the cylindrical small-diameter portion can be stably built in the other part.