1. A foreign matter inspecting method comprising steps of: irradiating an inspection area of an article with inspection light; comparing intensity of either reflected light or scattered light generated from the inspection area of the article; setting a threshold value for each inspection area of the article so as to determine whether or not foreign matter is present in said inspection area; associating an inspection result with coordinates information; associating the threshold value with the inspection result or the coordinates information; overlapping images of said inspection area; multiplying a coefficient by a variation of an image overlap; displaying both threshold levels of said article by said multiplying, and a detection sensitivity image indicated by being converted from said threshold levels of said article; setting a foreign matter detection upper limit value for each area of the detection sensitivity image; and determining whether each area of the detection sensitivity image exceeds the set foreign matter detection upper limit value.