Corner piece-less duct, method of producing the same, and duct-forming machine

A cornerplate-less duct which has a tubular shape with a rectangular cross-section, the duct comprising:

• a plurality of plate members joined to each other at side end portions thereof by seam joints, in which flange portions which are connecting faces adapted to connect adjacent ducts are formed integrally at a starting end portion and a terminal end portion of a plate formed into each plate member by bending the plate in a direction substantially perpendicular to a longitudinal direction of the plate, corner forming portions are formed to protrude outward from side ends of the flange portions in a width direction of the plate, and the corner forming portions of adjacent plate members are overlapped with each other to form corner portions of the flange portions of the duct in an assembled state;
• wherein the plate is structured such that a first cut portion is formed in a portion which is closer to a center in the longitudinal direction of the plate than a starting end or a terminal end of the plate such that the first cut portion extends in the width direction of the plate and has a depth substantially equal to an overlapping portion of the corner portion.