1. An optimum imaging position detecting method comprising: acquiring an image of a predetermined area of a substrate surface; calculating, on the basis of the image of the predetermined area and for each of a plurality of intensity acquisition parts included in the predetermined area, peak intensity corresponding to a value obtained by subtracting average signal intensity of an area outside the intensity acquisition part from signal intensity of the intensity acquisition part; calculating variation of the peak intensity calculated for each of the plurality of intensity acquisition parts; executing acquiring the image of the predetermined area, calculating the peak intensity, and calculating the variation of the peak intensity at each of a plurality of imaging positions; and determining, on the basis of the variation of the peak intensity obtained at each of the plurality of imaging positions, that a position of the maximum variation of the peak intensity is an optimum imaging position.