In the previous report, the strength of a horizontal reservoir supported at both ends and completely filled with a liquid was discussed, utilizing the fundamental differential equation introduced by Dr. K. Mizoguchi, and on comparing the results with those obtained by Timoshenko's method, it was pointed out that the latter's method, was inadequate. In this paper, the strength of the same reservoir, being partially filled with a liquid, is discussed on utilization of the same method as before. In this case, the behavior of the deformation of the shell is very complicated; e.g. the deflection is larger in a lower head than in a higher head of the liquid, which shows that the use of the approximate method such as that of simplified equation or the energy method is very dangerous.