To improve the strength at a butted part of a thick plate and a thin plate, and, in particular, to keep the strength at a corner part of a hatchway of a hull.

A joined structure is provided with a thin plate part 1c in which an edge part of a thick plate 1a is of the same thickness (t) as that of a thin plate 1 as far as the position slightly away from a joined part J on the thick plate 1a to form a corner part of a hatchway of a deck of a hull, and a part 1e in which the thickness is gradually increased to the thickness T of the thick plate body. Even when the stress concentration is generated due to the geometrical discontinuity at the boundary of the thin plate part 1c with the gradually- increased thickness part 1e, the structure is favorable in terms of the strength compared with the conventional structure because the corner part is away from the joined part J.