In a method of manufacturing a photo mask used in a semiconductor manufacturing process, a mask pattern layout in which a plurality of patterns are arranged is acquired. The plurality of patterns are converted into a graph having nodes and links. It is determined whether the nodes are colorable by N colors without causing adjacent nodes connected by a link to be colored by a same color, where N is an integer equal to or more than 3. When it is determined that the nodes are colorable by N colors, the nodes are colored with the N colors. The plurality of patterns are classified into N groups based on the N colored nodes. The N groups are assigned to N photo masks. N data sets for the N photo masks are output.