PURPOSE: To enable working narrow width sheet metal pieces excellent in accuracy by executing bending work pressurizing a piston-cylinder apparatuses in a region of a sheet metal piece to make pistons on both the outer ends touch stoppers, and removing the pressure on the other piston-cylinder apparatuses.

CONSTITUTION: Between a bending male die 3 and a bending female die 5 a bar shaped sheet metal piece 8 of narrow width is inserted. Two piston-cylinder apparatuses 7a, 7b below a sheet metal piece 8 is pressurized to hold the sheet metal piece 8. By operating hydraulic operating ram 2, the sheet metal piece 8 is subjected to air bending, and then a bending edge of the sheet metal piece 8 is made to touch the bottom of the female die 5. The piston-cylinder apparatuses 7a, 7b are pushed back by pressure, and the sheet metal piece 8 is bent by a uniform pressure along an elastic curve of an edge of the male die 3. At this time, if an air gap remains out of the region of the sheet metal piece 8, an unstable situation occurs, so the height is elevated by a sheet thickness h at the both ends 5' of the female die 5 to support the male die 3 stably. Even in a case of a sheet metal piece of an extremely narrow width, a precise working is possible without getting out of position to one side.