PURPOSE: To facilitate assembling of a thin plate square duct.

CONSTITUTION: U-shaped sockets 2a, 2b are formed at side edges of plate materials 2, 2 of a square duct formed by perpendicularly connecting thin duct side plates (plate materials) 2. Elastic connecting metals 3 each having a rectangular equilateral triangular section in which a pair of platelike plugs 3a, 3b to be inserted to the sockets 2a, 2b are integrally provided at both ends of a back surface 3b are inserted from one ends of the materials 2 to assemble a square duct. Thus, the duct can be simply assembled at a duct installing site. Since the metals 3 are formed of relatively thick steel materials, spring forces contained in the metals 2 are so operated via the pair of the plugs 3a, 3b as to bring the sockets 2a, 2b into close contact, thereby obtaining the duct having excellent rigidity and airtightness.