A layout design method for a semiconductor device includes a step of arranging transistors, a dummy gate forming step of forming dummy gates, which has a shape identical with a shape including gate electrodes or the gate electrodes and projected parts from active regions of the gate electrodes, in positions in parallel with and a fixed distance apart from the gate electrodes arranged at both ends in a gate length direction on active regions of the transistors and, when the transistors have plural gate electrodes with different gate widths, extending the projected parts to the outside of the active regions by a necessary length, a gate connecting step of, when gate patterns and contact regions are connected to the gate electrodes of the transistors, connecting the gate electrodes and the dummy gates according to a positional relation between the gate electrodes and the dummy gates, and a wiring step of wiring a metal layer. It is possible to design a semiconductor device having a smaller area than that in the past and with a less design man-hour.