PURPOSE: To prevent the deforming of opening caused with bending by bending the material to be formed to the prescribed shape so as to be bent and deformed with the opening in a flat plane in the state of the core material of elastic material being inserted in the opening of the shaped material, and next taking the core material out of the material to be formed.

CONSTITUTION: Because the bending is executed in the state of the core material 2 being inserted in the opening of the material to be formed, the opening is prevented from deforming during the bend working. Because the core material 2 is formed of the elastic material, there is no trouble for bending, and because the bending is executed in the state of the opening being positioned in the bending plane, it is easy to take out the core material of the material to be formed after working. Further, by utilizing the elastic plate material formed with the many grooves stretching in vertical direction against the longitudinal direction on the both side faces as the core material, the working can be easily executed without the generation of plastic deformation for the core material against the bending of large curvature.