In a defect inspecting apparatus, an illumination optical system illuminate a mask having a patterned surface, the optical beam passing through the mask is split into two beam components which is guided in first and second image pickup sensors. The pickup sensors has first and second pickup fields on the patterned surface, which pick up first and second parts of the mask image. The first and second pickup fields are parallel to each other and displaced from each other by (2n+1)×d/2 in the longitudinal direction thereof, where d denotes a longitudinal dimension of each pixel image in the first and second pick up fields and n denotes an integer equal to or larger than 0. The first and second parts of the mask image are merged to form a pattern image, and a defect in the mask is detected on the basis of the pattern image.