1. A photomask inspection method, comprising: acquiring reflected image data and transmitted image data of a photomask having a glass region and a film region formed on the glass on a surface thereof; creating inverted reflected image data by subtracting the reflected image data from pixel value data of the glass region and the film region; creating offset inverted reflected image data by raising pixel values of the inverted reflected image data by a fixed amount; creating normalized correlation reflected image data by computing a normalized correlation of the offset inverted reflected image data and an offset Gaussian distribution-type kernel; identifying foreign particle candidates by comparing the normalized correlation reflected image data and a predetermined threshold; creating offset transmitted image data by subtracting the pixel value data of the film region from the transmitted image data and then raising the pixels values by the fixed amount; creating normalized correlation transmitted image data by computing a normalized correlation of the offset transmitted image data and the offset Gaussian distribution-type kernel; identifying holes by comparing the normalized correlation transmitted image data and the predetermined threshold; and determining, among the foreign particle candidates, those foreign particles that are not located at the same position as the holes as true foreign particles.