PURPOSE: To improve the bending precision by engaging the engaging part of a side frame and the part to be engaged of an auxiliary arm in the condition of rotatably supporting an upper roll and preventing the deflection of the auxiliary arm in the front and rear direction.

CONSTITUTION: In the supporting posture for rotatably supporting the upper roll 1 by fitting the bearing part 121 of the auxiliary arm 12 to the tip end part of the upper roll 1, the engaging part 71 arranged at the side frame 61 and the part 72 to be engaged arranged at the auxiliary arm 12 are engaged at the upper part of a shaft supporting part 122. The relation between the engaging part 71 and the part to be engaged is constituted with the projecting streak extended to the vertical direction and the recessed groove closely fitted with the projecting streak or has the construction mutually continuing the projecting streaks and the recessed grooves having V-shaped cross section. In such a way, since the auxiliary arm for supporting the upper roll 1 is difficult to deflect before and after the bending work, the rotation of the upper roll 1 is smoothened and the bending precision can be held.