A mask inspection apparatus includes irradiation means for irradiating a sample with an electron beam, electron detection means for detecting a quantity of electrons generated from the sample having a pattern formed thereon by the irradiation with the electron beam, image processing means for generating image data of the pattern on the basis of the quantity of the electrons, and control means for creating a line profile and a differential profile of the pattern formed on the sample on the basis of the quantity of the electrons detected by the electron detection means. The control means detects a rising edge and a falling edge of the pattern on the basis of the differential profile, and then generates mask data of a multi-level structure on the basis of data of the edges and the image data created by the image processing means.