1. A foreign matter inspecting method for comparing intensity of either reflected light or scattered light, which is generated from an inspection area of an article to be inspected by scanning irradiation of inspection light, with a threshold value set to each of divided inspection areas so as to decide whether or not a foreign matter is present in said inspection area; wherein: both a threshold image in which said threshold value is indicated over an entire area of said inspection area, and a detection sensitivity image indicated by being converted from said threshold image can be displayed.