[Problems] To provide a duct flange structure which has a higher strength and which is easier to handle and manufacture.[Means for solving the Problems] A duct flange structure which is arranged in an end portion of a cylindrical duct body 2 having a substantially quadrilateral shape in section, includes: a flange portion 4 formed by bending an extension portion 30 extending from an opening edge 3A of each of wall plates 3 constituting the duct body 2, the flange portion 4 being constituted by a vertical portion 41 extending outside substantially vertically to the wall plate 3 and a horizontal portion 42 extending substantially parallel to the wall plate 3 from the vertical portion 41, the vertical portion 41 and the horizontal portion 42 being formed by folding and superposing the extension portion 30 in two or more.