An image processing device that accumulatively generates a synthetic image by joining a first image, which is an initially input image or is a previous synthetic image constituted by a plurality of previously input images that were joined together in one or more previous iterations, and a second image that is a newly input image, every time when a new second image is input, the device comprising: a center-position obtainment portion that obtains position information on one or more first center points of the one or more input images constituting the first image and position information on a second center point being a center point of the second image; and a synthetic image generation portion that obtains the first center points of one or more images overlapping the second image among the input images constituting the first image, and that joins the first image and the second image to generate the synthetic image, using one or more perpendicular bisectors between the one or more obtained first center points and the second center point as a joint between the first image and the second image, based on the obtained position information on the one or more first center points and the obtained position information on the second center point, wherein the synthetic image generation portion determines a pixel value at a predetermined position of the synthetic image based on a distance from a perpendicular bisector between the first center point being closest to the predetermined position and the second center point, wherein the synthetic image generation portion regards a position in grid points arranged in grid-arrayed points as the predetermined position and records the first center point being closest to the grid point for each of the grid points, and wherein the synthetic image generation portion determines pixel values within a block surrounded by the grid points based on the distance determined for each of the grid points. a center-position obtainment portion that obtains position information on one or more first center points of the one or more input images constituting the first image and position information on a second center point being a center point of the second image; and a synthetic image generation portion that obtains the first center points of one or more images overlapping the second image among the input images constituting the first image, and that joins the first image and the second image to generate the synthetic image, using one or more perpendicular bisectors between the one or more obtained first center points and the second center point as a joint between the first image and the second image, based on the obtained position information on the one or more first center points and the obtained position information on the second center point, wherein the synthetic image generation portion determines a pixel value at a predetermined position of the synthetic image based on a distance from a perpendicular bisector between the first center point being closest to the predetermined position and the second center point, wherein the synthetic image generation portion regards a position in grid points arranged in grid-arrayed points as the predetermined position and records the first center point being closest to the grid point for each of the grid points, and wherein the synthetic image generation portion determines pixel values within a block surrounded by the grid points based on the distance determined for each of the grid points.