1. A pattern layout designing method comprising: creating a graph in which mask patterns adjacent to one another at a distance in which desired printing resolution cannot be obtained in a lithography process or a distance in which an assist pattern for obtaining the desired printing resolution cannot be arranged among mask patterns generated based on a pattern layout design drawing are set as nodes connected to one another by edges; extracting closed loops from the created graph; selecting an odd number loop formed by an odd number of nodes from the extracted closed loops, arbitrarily selecting, when the selected odd number loop is isolated, rearrangement target nodes from the odd number loop, and selecting, when the selected odd number loop is not isolated, based on whether a closed loop group in which a plurality of closed loops including the odd number loop are connected includes an even number loop formed by an even number of nodes, rearrangement target nodes from the odd number loop included in the closed loop group according to different selection references; and rearranging layout of patterns described in the pattern layout design drawing corresponding to the selected rearrangement target nodes.