To obtain the working system restoring the torsional motion formed in a round material piece and juxtaposing the whole bend in the substantially the same surface by temporarily arranging the roll of a tensile device in the opening position not conflicting with and moreover not coming into contact with the round material piece in the direction crossing it before bending after finishing the positioning process and successively returning it to the closing position.

At the time of finishing the feeding and positioning of the round material piece preceding bending, the roll 15 of the tensile device 12 reaches the position not brought into contact with the roll 15 from the position 15 (I) closed by the roll 15 on the round material piece, and accordingly reaches the partial opening position 15 (II) of the chain line opening the round material piece. Any contrast and/or gripping means arranged in the upstream or downstream of the tensile device 12 are likewise opened to completely release the round material piece from contact in the lateral direction. Before executing bending, the roll 15 is re-arranged in the closing position 15 (I) to hold the round material piece and is acted as the contrast of bending.