A method of generating mask data, for a set of masks used to transfer a pattern for delineating a circuit pattern of a semiconductor integrated circuit, includes preparing design data having a design pattern corresponding to the pattern to be transferred on a semiconductor substrate; generating resized data by enlarging the design data by a resizing quantity; generating first mask data by filling a space area having a space width of a space quantity or less of the resized data; and generating second mask data, to be aligned with the first mask data, having a window portion for selectively exposing an area determined by enlarging the space area by the resizing quantity.