The construction of wooden ships is not regarded as a simple beam, but as a group of longitudinal members which are jointed together frames, deck beams, diagonal shell-planks using by nails etc. On the basis of this analogy, anthors find the method how to solve the problem on the strength and rigidity of built-up-beam, and the new way to reserch those of wooden ships. At the first stage, the authors solve the problem of the multi-layer built-up-beam using by infinite members of shear springs which join each beam and act to reduce the relative slip among each, layer. In other words, this spring action is quite similar to frames, etc. at actual ship. And solution by this process is proved accurate by the comparison with the results of bending-experiments on simplified models of wooden ships. We conclude that the strength of ships by ordinary construction system will be solved by this. process as well as other construction system which is adopted for modern wooden ships.