PURPOSE: To prevent particle from falling onto the belt in such a way that left and right side rollers can be tilted in the opposite directions at the same time by single operation of one operating rod.

CONSTITUTION: In the place over which a belt 3 rectilinearly runs, left and right side rollers 2 are kept to be held approximately at 60W80°, and in the place where the belt 3 becomes arciform, a butterfly screw 18 is loosened, then an operating rod 8 is pulled toward the operator setting a supporting shaft 7a as the fulcrum by grasping a grip 8b, and the side rollers 2 are tilted from the actual line position to the imaginary line position by setting a shaft 12 as the fulcrum. Since the side roller 2 on the right side and the side roller 2 on the left side are tilted in the opposite directions to each other, and since the belt 3 is prevented from one-sided ascent and one-sided descent by synergy of the both actions, particle will be avoided being fallen.