SURFACE DEFECT DETECTION APPARATUS AND SURFACE DEFECT DETECTION METHOD

First illumination light and second illumination light that are distinguishable from each other are each applied to an inspection target part of a surface of a steel material with substantially the same incident angle from directions inclined opposite to each other. A first image of the inspection target part illuminated with first illumination light and a second image of the inspection target part illuminated with second illumination light are each captured. A difference image between the first image and second image is generated. A combination of a bright part and a dark part of a protruding part in the inspection target part is removed from among bright parts and dark parts of the difference image based on an arrangement of a bright part and a dark part in a predetermined direction corresponding to the irradiation direction of the first or second illumination light. The presence or absence of a recessed part in the inspection target part is determined based on a shape feature amount of the remainder of the bright parts and the dark parts after this removal processing or an arrangement of the remainder of the bright parts and the dark parts in the predetermined direction. The recessed part that has been determined to be present is detected as a surface defect of the steel material.