PURPOSE: To provide a shell plate stiffener assembling method which requests no fitting to a shell longitudinal member and no welding work of a reinforcing plate between a hull shell plate and an outer bracket, hardly generates a deformation owing to welding distortion, and can facilitate assembling.

CONSTITUTION: At the horizontal edge and the vertical edge of an outer bracket 4 which provide a section surrounded by an inner bottom plating 7 and a side girder 5, at the inner side of a curved shell plate 1, cut-out parts 9 and 10 fitting to an inner bottom plate longitudinal member 8 and a side girder longitudinal member 6 are provided. A cut-out part 11 continuing to the height of shell plate longitudinal member 2 is provided at the curved edge of the outer bracket 4, and the outer bracket 4 is welded to the lower side of the inner bottom plate 7 by fitting the cut-out part 9. The side girder 5 is welded by fitting the positions of the cut-out part 10 to the vertical edge of the outer bracket 4, and the shell plate 1 to which the bracket 3 between longitudinal members is welded so as to block between projecting shell longitudinal members 2 is welded by fitting the cut-out part 11 of the curved edge of the outer bracket 4 integrated with the inner bottom plate 7 and the side girder 5.