In order to make a system safety and reliability assessment of ship structures in the longitudinal strength point of view, it is essential to analyze ultimate longitudinal strength of ship's hull girder under sagging or hogging bending moment. In the previous report, the author formulated an idealized plate element using idealized structural unit method and the developed method was then applied to ultimate longitudinal strength analysis of unstiffened hull structure as an example, concluding that the method is useful for ultimate collapse strength analysis of plate structure.In this study, an attempt is made to analyze ultimate longitudinal strength of stiffened hull structure by using idealized structural unit method. For this purpose, an idealized stiffened plate element subjected to biaxial load is developed taking account of the influence of initial imperfections as well as the interaction effect between local and global failure in the structure. The developed method is then applied to ultimate longitudinal strength analysis of Suezmax-sized double skin hull girder as an example for the intact and the damaged condition in the event of accidents of grounding and collision. A new deterministic measure of safety based on the absorbed energy capacity is proposed. Based on the obtained results, ultimate longitudinal strength-based safety and reliability assessment is made using the proposed measure of safety in addition to the conventional one, concluding that the objective hull structure has relatively sufficient safety and the present procedure is useful when ultimate longitudinal strength-based safety assessment or optimization is made in the structural design stage for a new type of hull structure.