Abstract: Regular-size white cells are expanded in advance to adjust a boundary between black cells and the white cells before a printing operation is performed. Shrinkage of black cells occurs in the region where the expanded white cells overhang over the neighboring black cells. No shrinkage of the black cells occurs in the boundary between two black cells which are located next to each other. A region where one white cell overhangs over another white cell remains white. This is simply realized by outputting 0 for the region where the white dot (0) and the black dot (1) are overlapped and by outputting 0 for the region where two white dots (0) are overlapped. By shrinking the black cells beforehand in this manner, the overhang of the black cells over the while cells is reduced or completely eliminated when they grow fat through the data reading performed by facsimile or copying machines.