PURPOSE: To prevent the inside surface of the opening groove of the work from being damaged by composing the contacting surface part for bending of the core metal of the elastic supporting body in the bending device for long sized work.

CONSTITUTION: On the bending work device consisting of respective devices 1, 3, 5 of the work feeding, bending and supporting for the long sized work W (channel steel) having the fixed cross section and the opened groove 101, the bending member 29 of rigidity which is brought in contact with the outer surface of the work W at the proximity of the bending operating spot and moves by shifting the position to the direction crossing to the longitudinal direction thereof and the core metal 40 to insert into the opening of the work W are set to the bending device 3. The core metal 40 is the elastic supporting body having low friction coefficient bending hard to deform in volumetric easticity, and forms the bend contacting surface part 42. Therefore, the core metal 40 is elastically deformed by the bending pressurized contract with the work W, so the pressing force is absorbed and the generation of damage of the inside surface of the opening groove 101 of the soft work W in contact with the elastic supporting body is prevented.