In a first process of a pattern generation method, a first segment to be handled which is not on a grid is extracted. In a second process, a second segment opposite to the first segment is extracted. In a third process, whether the second segment is on the grid is determined. In FIG. 1A, the second segment is not on the grid. Therefore, in a fourth process the first segment is shifted onto the grid under a determined condition. In addition, the second segment is shifted onto the grid so that line width between the first segment and the second segment is closest to target line width.