An objective of the present invention is to provide a joint structure of a duct that can be formed easily and comprising features of the joint structure of a novel duct previously filed by the present applicant. A tip end portion of one of two plate members is bent to form an angle of 90 degrees or more, a U-shaped groove is formed at an tip end portion of the other plate member such that at least an opening portion of the groove is adapted to a bending angle of the one plate member so as to accommodate the bent tip end portion of the one plate member in an end portion of the other plate member, and the guide rail is formed by protruding at least a portion on a bottom side of the U-shaped groove of the other plate member obliquely from an outer face located on a base end side of a portion where the U-shaped groove is formed and constituting one of the wall faces of the duct.