A method of recycling a large-size photomask substrate, comprising the steps of: (i) removing a patterned light-shielding film from a used large-size photomask substrate to provide a large-size photomask-forming glass substrate stock having a diagonal length of at least 500 mm and a thickness of at least 3 mm to be regenerated, (ii) resurfacing the glass substrate stock using a processing tool of sand blasting, (iii) repolishing the resurfaced glass substrate stock to yield a regenerated glass substrate stock, (iv) applying a light-shielding film onto the regenerated glass substrate stock to yield a regenerated large-size photomask-forming blank, and (v) processing the light-shielding film of the blank into a pattern corresponding to the desired exposure of a mother glass, yielding a regenerated photomask substrate; said step (ii) of resurfacing by sand blasting comprises the step of processing the large-size photomask-forming glass substrate stock having front and back surfaces, by removing therefrom a flattening removal quantity of material based on height data of the flatness and parallelism of the front and back surfaces of the large-size glass substrate stock in the vertical attitude, plus a deformation-corrective removal quantity of material, through sand blasting, the deformation-corrective removal quantity being calculated from a deflection of the glass substrate stock by its own weight in the horizontal attitude, calculated from the thickness and size of the glass substrate stock, and the support position when the regenerated photomask substrate is horizontally supported, a deformation of the regenerated photomask substrate caused by the photomask substrate support when the regenerated photomask substrate is attached to the exposure apparatus, and an accuracy distortion of a platen for supporting the mother glass to be exposed, the regenerated large-size glass substrate resulting from the glass substrate stock has an arcuate shape in cross section, such that the surface to be opposed to the mother glass is concave when held vertically, to reduce the proximity gap variation between the mother glass and the regenerated photomask substrate when held horizontally with the opposed side edges of the regenerated photomask substrate supported in the exposure apparatus, and wherein the regenerated photomask substrate is to be used in a mother glass exposure process including attaching the regenerated photomask substrate having opposed sides to an exposure apparatus through support of opposed side edges, disposing adjacent to the regenerated photomask substrate a mother glass serving as an array side or color filter side substrate in a TFT liquid crystal panel, and irradiating light from the exposure apparatus to the mother glass through the regenerated photomask substrate. (i) removing a patterned light-shielding film from a used large-size photomask substrate to provide a large-size photomask-forming glass substrate stock having a diagonal length of at least 500 mm and a thickness of at least 3 mm to be regenerated, (ii) resurfacing the glass substrate stock using a processing tool of sand blasting, (iii) repolishing the resurfaced glass substrate stock to yield a regenerated glass substrate stock, (iv) applying a light-shielding film onto the regenerated glass substrate stock to yield a regenerated large-size photomask-forming blank, and (v) processing the light-shielding film of the blank into a pattern corresponding to the desired exposure of a mother glass, yielding a regenerated photomask substrate;