An image processing apparatus comprising: image obtaining means for obtaining a first image and a second image in which a subject is captured; search area size calculation means for calculating a search area size; 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 first search area made up of the search area size in a surrounding of the correspondence position on the second image; 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 first search area on the second image; deformation information obtaining means for obtaining deformation information between the first image and the second image; and difference image generation means for generating a difference image which comprises difference values calculated at multiple respective positions of interest, wherein: the position obtaining means obtains the correspondence position corresponding to the position of interest on a basis of the deformation information, and the apparatus being characterized in that: the search area size calculation means is configured to calculate the search area size on the basis of a pixel size of the first image and the second image, and to obtain a constant to be multiplied with the search area size on a basis of an obtaining method for the deformation information. image obtaining means for obtaining a first image and a second image in which a subject is captured; search area size calculation means for calculating a search area size; 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 first search area made up of the search area size in a surrounding of the correspondence position on the second image; 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 first search area on the second image; deformation information obtaining means for obtaining deformation information between the first image and the second image; and difference image generation means for generating a difference image which comprises difference values calculated at multiple respective positions of interest, wherein: the position obtaining means obtains the correspondence position corresponding to the position of interest on a basis of the deformation information, and the apparatus being characterized in that: the search area size calculation means is configured to calculate the search area size on the basis of a pixel size of the first image and the second image, and to obtain a constant to be multiplied with the search area size on a basis of an obtaining method for the deformation information. the position obtaining means obtains the correspondence position corresponding to the position of interest on a basis of the deformation information, and the apparatus being characterized in that: the search area size calculation means is configured to calculate the search area size on the basis of a pixel size of the first image and the second image, and to obtain a constant to be multiplied with the search area size on a basis of an obtaining method for the deformation information.