To prevent biting into first and second rollers by separating a third roller and a fourth roller from each other without any increase in manufacturing cost.

A holder 20 mounts the third and fourth rollers 3, 4 around a bearing 22 and also surrounds the periphery thereof. A coiled spring 32 is set on the holder 20, and presses through a washer 33 by a bolt 34 to adjust the range of applying pressing force thereto. A coiled spring 41 having lower load than the pressing force is set on two holders 20 to apply separating force smaller than the pressing force in the opposite direction to that of the pressing force.