A device for forming seam of elbow duct is provided, and the device for forming seam of elbow duct that molds a lockseam by bending a flange of one pair of adjacent unit members, of a plurality of unit members constituting the elbow duct, including a base that supports a lower portion of the one pair of unit members; an elevating part that moves the base up and down; an angle adjustment part that adjusts a supporting angle of the base; a fixing chuck that supports an inner circumferential surface of a connecting portion of the one pair of unit members; a rotating part that rotates the fixing chuck; and a forming part that bends the flange of the one pair of unit members in a state where the flange is in contact to each other, to form the seam.