To protect an idle roller from going over clearance between an input and an output rollers (first and second rollers), by limiting the displacement of an idle roller (third and fourth rollers).

A first roller 1 and a second roller 2 are equipped with two shafts a, b so that they do not contact each other, and a third roller 3 and a fourth roller 4, which contact both of the first and the second rollers 1, 2 are arranged between the first roller 1 and the second roller 2 in relation to the specified angle of friction. A backup roller 30 for limiting the displacement of the third and the fourth rollers 3, 4 to the specified volume is equipped being in contact with the third and the fourth rollers 3, 4.