PURPOSE: To provide a working device for forming a folded flange by folding the tip part of a duct fixing an angle flange.

CONSTITUTION: In the folding device for flange, folding so as to bring an offset 8 of the duct 1 fixing the angle flange 5 into close contact with the angle flange 5 by arranging the offset 8 sticking out the tip part of the duct 1 by a prescribed length, a shifting frame 10 being turnable while using a shaft 12 fixed to the one end of a housing 11 as a fulcrum and shiftable to the front and the rear sides by energizing with a coil spring 25 is provided. A roll 14 for bending the offset 8 is arranged at the one end of the shifting frame 10. An angle flange abutting part 11b is arranged at the back side of the other end edge 11a of the housing 11. The shifting frame 10 is rotated by working an air cylinder 16 connected through a link 18 and a connecting rod 15, and the offset 8 is bent with a roller 14 for working to manufacture the folding flange.