Strength of the shell subject to external pressure is experimentally and theoretically studied for an about 70m long wooden ship with triple skin planking which consists of the longitudinal planking and the first and the second layers of diagonal planking tied by fixing bolts. The 1/2-scaled wooden model was subjected to the static pressure up to 2.0 kgf/cm² assumed as a probable maximum pressure, the 15, 000 times of low cycle fluctuating pressure between 0 and 2.0 kgf/cm², and the destructive pressure gradually increased up to 9.5 kgf/cm², followed by the three-dimensional (3D) frame analyses and some beam analyses to find a reasonable way of estimation. The results are as follows : (1) No damage was found at the static and the fluctuating load tests, confirming the structure has sufficient static and fatigue strength. (2) No rapid collapse occurred in the range of the tested destructive pressure. Only slightly bent necks were found at a fairly large number of the fixing bolts when examined after the test. (3) The deflections and stresses by the experiment are expressed fairly well by the 3D-frame model (3) which takes into account the elastic deformation of fixing bolts and also the lateral compression of frames within their width. Shear rigidity due to a group of fixing bolts is expressed by a bending beam element, of which the effect is found considerably large. (4) The deflectional mechanism of the triple skin planking is well explained by the newly introduced diagonally-crossing continuous beams, and also by the single continuous beam which gives results equivalent to those by the former. These two models give results similar to those by the 3D-frame model (2) which is before adding the effect of fixing bolts to the model (3).