1. A mask forming method comprising: preparing design data of a mask including a plurality of pattern regions, the plurality of pattern regions including identical repetition patterns, respectively; generating mask pattern data of the mask based on the design data; generating inspection control information for controlling inspection of a defect on the mask based on the mask pattern data, the inspection control information including positional information of the plurality of pattern regions and inspection sensitivity information of the repetition pattern; providing the inspection control information to the mask pattern data; forming a mask pattern of the mask based on the mask pattern data to which the inspection control information is provided; and inspecting the mask pattern based on the mask pattern data to which the inspection control information is provided comprising inspecting a portion in the mask pattern being different from the plurality of pattern regions by Die-to-Database comparison method, the inspecting the portion in the mask pattern being different from the plurality of pattern regions by Die-to-Database comparison method including selecting a portion corresponding to the repetition pattern in the plurality of pattern regions from the mask pattern based on the positional information of the plurality of pattern regions; and inspecting the selected portion by Die-to-Die comparison method at an inspection sensitivity corresponding to the inspection sensitivity information.