A necessary signal is kept on a difference image, and also, it becomes possible to reduce noise. An image processing apparatus includes search area calculation means for calculating a search area size on the basis of a pixel size of at least one image among a first image and a second image, position obtaining means for obtaining a position of interest on the first image and a correspondence position on the second image corresponding to the position of interest, search area setting means for setting a search area made up of the search area size in a surrounding of the correspondence position on the second image, and difference means for deciding a difference value corresponding to the position of interest on a basis of a density value of the position of interest on the first image and density values of a plurality of positions in the search area on the second image.
