Recenty, an oil tanker is becomming largar and larger and now, the enormous large tanker with over 300, 000 D. W. tons has being planed. In designing such a large tanker, it is important to determine the most efficient type and the scantlings of the midship section. For the basic study of this problem, authors tried to examine the effectiveness of the side girders attached on the bottom and the deck part both in a centre and wing tanks, that is, (1) to deduce the method of the three dimensional strength calculation for the with side girder type tanker, expanding the method explained in our previous paper, (2) to examine the effect of the side girders upon the transverse strength and on the longitudinal strength and to examine the strength of the side girder itself. Form this study we obtained the following results, that is (1) the side girders are not effective for the transverse strength but effective for the longitudinaly bending action, (2) the shearing stress of the side girder becomes much larger in the case of the rigidity of the side girder is smaller than those of the transverse members, on the contraly, the normal stress of the side girder becomes larger in the case that the rigidity of the side girder is larger than those of the transverse members, (3) it seemes to us that, it is, therefore, better not to attach the side girders on both the bottom and the deck part, and moreover, if necessary, the _??_-giders may be removed for the sake of the simplicity of the hull structure.