The bottom plates of cylindrical flat-bottom tanks such as on ground LNG storage tanks generally have detached parts from the foundation due to the welding deformation. And the allowable limit of it from the view point of the strength under the liquid pressure becomes an important problem at the actual construction of tanks. However, there have been few reports which mentioned this problm. In this paper, the results of theoretical study on this problem, assuming the welding deformation is two dimensinonal, are presented. The behaviors of the detached bottom plate under liquid pressure are found as follows ; (1) When the liquid pressure begins to exert on the bottom plate, the major part of it touches to the foundation and the considerable amount of bending of the plate occurs in the detached part. (2) When the liquid pressure reaches to some level, the whole bottom plate touches to the foundation completely in an unstable manner. The proposal for the method of determining the allowable welding deformation is also presented.