To prevent a holder from being damaged by retaining the holder at a set position.

The holders 20a, 20b can be rocked around a rocking pin 23 corresponding to the side movement of third and fourth rollers 3, 4. At this time, since the positions of accommodation sections 26a, 26b shift each other, a coil spring 25 is elastically compressed corresponding to the shift. As a rocking range increases, elastic return force is accumulated largely. When the rocking of the holders 20a, 20b is stopped, the holders 20a, 20b can be returned to the set position due to the elastic return force being accumulated in the coil spring 25.