A method of fabricating a photomask used to form a lens. The method includes the steps of generating mask pattern data for each of a plurality of grid cells constituting a mask pattern for the lens, and fabricating the photomask based on the mask pattern data. The step of generating the mask pattern data includes acquiring data which represents a transmitted light distribution required for the photomask to fabricate the lens, in which the transmitted light distribution includes a quantity of transmitted light in each of the plurality of grid cells, and determining whether to place a shield on each of the plurality of grid cells by binarizing the quantity of transmitted light in each of the plurality of grid cells in order of increasing or decreasing distance from a center of the mask pattern using an error diffusion method.