1. A method for generating a pattern of a cell used for a mask manufacturing method, the method comprising the following step executed by a processor: obtaining data of a cell including a plurality of pattern elements and data of a range of the cell; calculating optical images of the pattern elements of the cell in the inside and the outside of a range of the cell using the obtained data of the cell; generating an assist pattern which assists resolution of the pattern elements in the inside of the range of the cell using the calculated optical images of the pattern elements of the cell and the obtained data of the range of the cell; generating a pattern including the pattern elements and the generated assist pattern as a cell pattern; and storing the generated cell pattern in a cell library, wherein a mask pattern is generated by arranging cells selected from a plurality of types of cells stored in the cell library, wherein the mask manufacturing method comprises a step of manufacturing the mask including the generated mask pattern.