The longitudinal strength of wooden ships with flat bottom and wall side is solved by applying the new method which has already been described in the 1st. and 2nd. reports. But the rise of floor at usual wooden ships is not neglectable. In this report, the method to solve the strength of wooden vesseles with such sectional form, which is not rectangle but pentagon. In detail, obserbing the characteristic of wooden constructions with aforesaid sectional form, it is found the characteristic is not equal to that of single multi-layer built-up-beam, but equal to that of the built-up-beam which is constructed by two built-up-beams, the one built-up-beam correponds to upper deck and side shell part and another to bottom shell part. And then author find the method to annalise the strength of such built-up-beam. The solution by this way is proved its accuracy by comparison with the result of bending test on a model of wooden ship. Thus the fundamental part of the new method to solve the longitudinal strength of wooden vessels of usual form, is almost settled down with no limit in their construction system.