It is judged whether or not an average gray level of an image of a wafer W that is an inspection target and that has been imaged by the light receiving part 2 is in the defect detectable range. A control processing part 6a is configured to modify an exposure time in imaging the wafer W and to obtain an image of the wafer W again by the light receiving part 2 in the case in which it is decided that an average gray level of an image of the wafer W is not in a defect detectable range, and an image processing part 6b is configured to carry out a defect inspection based on an image of the wafer W in the case in which it is decided that an average gray level of the image of the wafer W is in the defect detectable range.