A layout design method of a semiconductor integrated circuit includes degenerating a layout netlist extracted from layout data, comparing the layout netlist after the reduction with a circuit diagram netlist, and creating a layout circuit association table of a layout cell after the reduction and a circuit element. The method includes creating a before and after reduction association table based on the layout netlist before and after the reduction, counting the number of layout elements in a layout cell area before the reduction, comparing the counted number of layout elements and the number of degenerated elements, and creating mapping information associating the layout cell with the circuit element.