Provided is a defect detection device (1) capable of measuring the volume of surface defects. The defect detection device (1) includes: an imaging device (3) configured to image an image of an inspection object; a binarization processing unit (11) configured to subject the image to first and second binarization processing by use of different first and second binarization thresholds, so as to calculate first and second sizes for an identical defect in the image; a ratio calculation unit (12) configured to calculate a first ratio of the second size to the first size; and a depth determination unit (13) configured to determine a depth of the defect depending on the first ratio.