A method for manufacturing a semiconductor device includes designing a layout, manufacturing a photomask based on the designed layout, and performing a photolithography process using the photomask to form a pattern on a substrate. The manufacturing of the photomask includes preparing the photomask including first and second chip regions, extracting first and second images from the first and second chip regions, respectively, averaging the first and second images to generate a preliminary standard image including a difference region between the first and second images, inserting a normal image into the difference region based on the layout to generate a standard image, and comparing each of the first and second images with the standard image to detect a defect of the first and/or second chip regions.