A photomask inspection method that identifies a foreign particle such as dirt on a photomask with high sensitivity by suppressing erroneous identification due to an influence of noise is provided. The photomask inspection method includes acquiring image data of a photomask having regions with different layer structures on a surface thereof, creating inverted image data by subtracting the image data from pixel value data of the regions, creating offset inverted image data by raising pixel values of the inverted image data by a fixed amount, creating normalized correlation image data by computing a normalized correlation of the offset inverted image data and an offset Gaussian distribution-type kernel, and identifying foreign particles by comparing the normalized correlation image data and a predetermined threshold.