Surface defect detecting method and surface defect detecting apparatus

A surface defect detecting apparatus that optically detects a surface defect of a moving steel material, the surface defect detecting apparatus comprising:

• an irradiation unit configured to irradiate an examination target part with illumination light beams from different directions with two or more distinguishable light sources whose light emission durations are set based on at least an allowable positional displacement of an image, the two or more distinguishable light sources repeatedly emit light such that their light emission timings do not overlap each other;
• and
• a detection unit configured to obtain images by reflected light beams of the respective illumination light beams and detect a surface defect in the examination target part by executing subtraction processing between the obtained images.