1. A mask defect inspection apparatus, comprising: an illumination optical system for illuminating a mask on which a pattern is formed; an objective lens facing said mask; at least a pair of detection optical systems each having a detection sensor for obtaining an image of the pattern of said mask, and being adapted to receive illumination lights from illumination areas different from each other through said objective lens, respectively; and a focusing changing device for changing a position of focusing between sites of the pattern in a film-thickness direction of said mask and the pattern images obtained by each of said detection sensors, such that pattern images obtained by each of said detection sensors are changed corresponding to the film-thickness direction of said mask.