A method for inspecting a uniformity of CD (CD) of a photo mask pattern increases a production yield. The method obtains a CD by precisely measuring a photo mask by using, an electron microscope. Then, a measurement image having, a plurality of patterns formed in the photo mask is obtained by photographing the photo mask at a high speed through an optical microscope. A gray level based on the CD is calculated by capturing just a pattern area in the measurement image, and an estimated value and a correlation coefficient is obtained, when an open density of the measurement image is relatively low. Accordingly, a uniformity of CD can be confirmed more clearly in a measurement of high speed for a measurement image having a relatively low open density.