1. A mask inspection apparatus comprising: 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; storage means for storing therein the image data; and control means for calculating a number of divided images, which form an entire combined image, on the basis of a size of a specified observed area of the sample, determining divided areas in such a way that divided images adjacent to each other overlap with each other, acquiring the divided images of the respective divided areas, and storing the divided images in the storage means, wherein the control means extracts two divided images adjacent to each other in a predetermined sequence starting from a specified one of the divided images of the respective divided areas stored in the storage unit, for each of the two divided images adjacent to each other, the control means then detects an image of a same pattern formation area included in an overlap area between the two divided images adjacent to each other, and determines the detected image to be a combination reference image, and the control means then combines the two divided images adjacent to each other on the basis of the combination reference image to thereby form an entire SEM image of the observed area.