PURPOSE: To bend a grooved strip material easily and correctly without deforming and damaging the same in the stage of bending said material by setting spring leaves in the grooves in the working position.

CONSTITUTION: Spring leaves 3aW3g of prescribed length are arrayed and set in grooves 2 around the position 4 to be bent of a grooved strip material 1 from one side 5 to the other side 6. Thence, in this state, the material 1 is bent and is passed in rolls 7W11. As a result, the inner side 5 is shrunk and the outer side 6 is extended, whereby the material is bent. In this case, the inner side 5 and the plate 3a, the plate 3 groups each other and the plate 3g and the outer side 6 back up each other respectively, and these are one body as a whole; therefore the external bending forces exterted to these act eqally to each part, thereby maintaining balance with the stresses produced from the respective parts. Hence, the local bending by the partially acted forces is eliminated and the strip material 1 is correctly bent.