In order to make accurate calculation of the transverse strength of ships, it is necessary to take account of the effects of longitudinal members, variable cross-sections of structural members, shear deflecion, etc., but the introduction of these effects would make the calculation very complicated. This paper deals with the approximate method of calculation by using diagrams to simplify the calculation. Investigations have been made here of the effect of longitudinal members upon the transverse members of cargo ships, and the effects of variable cross-sections of structural members and of shear deflection will be reported on the next occasion. When the transverse strength is considered in three dimensions, having regard to the effects of centre and side girders, deck girders, etc., transverse members in the middle part of the hold are the weakest, and therefore the analysis has been concentrated to the middle part of the hold. The results of the calculation according to this method have indicated fair agreement with those of the precise calculation, so that it is considered that this method has sufficient accurity in practical use. The author considers that this method, being simple in calculation, will be useful when strength calculation is made as a routine work.