A computer sets a process area based on an image obtained by observing a mask, and determines the positions of representative points that form a contour of the process area for each pixel with sub-pixel accuracy that is better than a pixel, the position of each of the representative points being able to be set to either the center position of the pixel or a position displaced therefrom. Furthermore, for the pixels within the process area, the computer sets the center positions of the pixels as the representative points and corrects the positions of the representative points of the pixels within the process area on a sub-pixel basis such that nonuniformity between the representative points is reduced. When the mask is processed, the charged particle beam is applied with sub-pixel accuracy to the positions of the representative points that form the contour for the pixels that includes the contour of the process area and to the positions of the corrected representative points for the pixels within the process area.