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.