PURPOSE: To prevent earth and sand from infiltrating into an idler bracket by providing an earth and sand infiltration-preventing cover in a yoke, and sliding this preventing cover by a predetermined displacement amount.

CONSTITUTION: A bracket main unit 42 of an idler bracket 41 is formed such that an upper plate part 42A is extended in a flat plate shape in a height position of approaching an upper end edge of an idler 19. A sediment infiltration- preventing cover 45, slid along a lower surface of the upper plate part 42A of the bracket main unit 42 to cover a clearance A between a guide hole 42D and the idler 19 from a lower side, is provided in a yoke 18. Even in the case of sliding the idler 19 in the idler bracket 41, a part between the guide hole 42D and the idler 19 can be always closed.