An object of the present invention is to provide a defect detection method and its apparatus which can adjust sensitivity easily by managing both reduction in the number of false reports and highly-sensitive detection of a defect using single threshold value setting in comparison inspection that compares an image to be inspected with a reference image to detect a defect judging from a difference between the images. According to the present invention, adjusting the brightness before inspection so that a difference becomes small at the edges of a high-contrast pattern in a target image enables reduction in the number of false reports caused by an alignment error, and achievement of highly-sensitive defect inspection using a low threshold value, without increasing the threshold value. Moreover, in the case of inspection targeted for a semiconductor wafer, which produces a problem of irregularity in brightness of a pattern caused by a difference between film thicknesses, the present invention reduces the number of false reports caused by the irregularity in brightness, and realizes inspection with high sensitivity without increasing a threshold value, by adjusting the brightness of both images before the inspection.