Recently, wooden ships with outer boards of double diagonal system were often constructed, and it was proved that they showed higher rigidity in comparison with ordinary wooden ships. In order to investigate the rigidity of a board of double diagonal system, a series of model experiments were carried out, and the results were analysed with the following method. The authors calculated the displacement of an elemental plank of the board by the “minimum energy theory” under the assumption that the elemental plank of the board were perfectly rigid and then the working energy was conserved in rivets alone, as the wooden structure with rivets shows a large slip between their constructional members. In this paper, the authors deal with the case under a tensile force. The calculated values of the displacement are compared with the experimental values with good results, as shown in Fig. 4. The distribution of shearing forces acting on the rivets is tabulate in Table 1.