We discuss the mechanism of image patterns of the frictional-force microscopy (FFM) of a graphite surface by using a three-dimensional model comprised of a tip connected to a cantilever and a substrate surface. A simulated FFM image is in good agreement with an experimental one. A stable domain of the tip atom position can be defined in an analytic way. In the frictional-force regime, more than one quasistable tip atom position are mapped into a single cantilever basal position. Part of the boundary of the two-dimensional domain of the cantilever basal position appears as a fringe between the bright and the dark areas along the scan direction of the FFM image. General features of FFM images can be completely understood by this analysis.