The determination of the longitudinal strength of ships is one of the most important subject in the design of ships. Moreover this determination must be made in the early design stage, and accordingly the longitudinal bending moment, which is a predominant factor affecting the longitudinal strength, should also be calculated in the early design stage. There is still a dearth of knowledge at present as to the standpoint from which the longitudinal strength should be determined, but the most common concept prevailing at present may be that the degree of safety for the longitudinal strength should be kept constant for all sea-going ships. This idea that the degree of safety for longitudinal strength be kept constant has such suggestions that, when account is taken of the fact that ships will encounter rough seas of different severity in various loading conditions, the longitudinal bending moment having statistical significance rather than the one by precise calculation for specific conditions should be used as the longitudinal bending moment for the purpose of discussing the longitudinal strength of ships. From the above view-point, the author developed approximate formulas for the longitudinal bending moment which would be used in the early design stage with sufficient accuracy. The longitudinal bending moment of ships can be divided to (a) the still water bending moment produced as a result of difference in the distributions between the weight and buoyancy in still water, and (b) the wave bending moment produced as a result of the change of the pressure distribution of underwater surface of the hull when the ship proceeds among waves, as well as the inertia force due to the ship's motion. The paper deals with the still water bending moment in the Part I and with the wave bending moment based on the statistical method in the Part 2. The result of investigations in Part 2 indicate that the approximate formulae used up to the present may, in general, result in the over-estimation of the bending moment in the case of super tankers in comparison with ships of moderate size. This conclusion will give usefull suggestion to the possibility of reducing the standard of longitudinal strength for super tankers.