PURPOSE: To perform accurately axial line bending without bulging the axial line of a material by dividing a retainer arranged at the front of bending rolls into right and left sides and moving either of both sides from its original position to the rocking direction of rolls by a connecting member.

CONSTITUTION: A retainer 10 consisting of roll groups 11c, 12c, etc. arranged oppositely on the right and left sides is divided to right and left, and is connected to bending rolls movably in accordance with the rocking direction of rolls by means of a connecting plate 40. A roll device 20 is rocked around a rocking-center axis 25 along the cam groove 29b of a slide plate 28. With this rocking, a roll 20a moves away from the retainer 10, and at the same time, one side of the retainer 10 moves out of its original position to the roll 20a side. In this way, the axial line bending is stably and accurately performed without bulging the axial line of a material stock.