The greater part of curved ship-hull is consisted of assembly of thin shallow shells reinforced with stiffeners. Static strength of shells and stiffened shells is analyzed by means of the finite strip method¹⁾ proposed by author in addition to the Fourier's analysis and the effects of stiffeners fitted for the purpose of stress reduction on shell plate are clarified. Therewith, the practical formulas for determining optimum stiffener for given shell are obtained. As results of investigation on stiffened shells, it is found that the less stiffeness of shell itself, the more effective of stiffener for strength of shell structure for the case of fitting a few stiffners.