A method of manufacturing a photomask set includes: preparing a mask layout, the mask layout including a plurality of first layout patterns apart from one another in a first region, wherein distances between center points of three first layout patterns adjacent to one another from among the plurality of first layout patterns respectively have different values; grouping pairs of first layout patterns, in which a distance between two first layout patterns adjacent to each other does not have a smallest value, and splitting the mask layout pattern into at least two mask layouts; and forming a photomask set including at least two photomasks each including a mask pattern corresponding to the first layout pattern included in each of the mask layout patterns split into at least two mask layouts.