Recently Frictional-Force Microscopy (FFM) has enabled us to measure the friction on an atomic level, and has opened a new research area in Micro-Tribology. However, from the theoretical standpoint of view, the question of what physical quantity is observed by FFM has not been fully clarified yet. Therefore we introduce a theoretical method of interpretation of two-dimensional frictional-force images based on numerical simulation and experimental analysis. As a case study, cleaved graphite surface is adopted as a sample surface. Simulated images reproduce experimental images quite well. We clarify that the part of the stable domain boundary of the cantilever basal position appears as a fringe between the bright and the dark area along the scan direction in the FFM image. Furthermore it is also clarified how the FFM images are influenced by the macroscopic conditions such as the load, scan direction, and anisotropy of the cantilever. By this analysis, physical meaning of the FFM image patterns can be clearly understood together with that of the stick-slip motion of the tip atom.