A cornerplate-less duct includes plate members joined to each other at side end portions by a seam joint to form a tubular duct having a rectangular cross-section. Two end portions of a plate are bent perpendicular to a longitudinal direction of the plate to form flange forming portions. Corner forming portions protrude from side ends of the flange forming portions in a width direction to form corner portions at the end portions of the duct. In an expanded shape of the plate, a cut portion is provided close to a longitudinal center of the plate member such that the cut portion extends in a width direction of the plate member and has a depth substantially equal to an overlapping portion of the corner forming portion.