A design method places a dummy line in floating state in a line layer of a semiconductor apparatus by using a computer. The method includes a first step of reading layout data and placing a dummy line with a longitudinal side lying in parallel with a signal line in an area where a pattern density of the signal line in a prescribed area is equal to or lower than a density lower limit, and a second step of dividing a dummy line placed in an area where a distance from the signal line is equal to or shorter than a dummy dividing distance.