An idler comprises an idler tube (1), an idler shaft (7), a bearing seat (2) and a labyrinth seal structure at each end of the idler tube (1). The labyrinth seal structure comprises a seal ring (5) and a seal cover (3). A positioning flange (12) is provided on the top edge of the bearing seat (2). A convex ring (4) and a clearer (9) are provided in the seal ring (5). A seal rim (3-1) of the seal cover (3) is fitted between the convex ring (4) and the clean device (9).