1. An image processing device that accumulatively generates a synthetic image by joining a first image, which is a single image or is constituted by a plurality of the images joined together, and a second image that is input, the device comprising: at least one processor that executes: a center-position obtainment portion that obtains position information on a first center point being a center point of each of the 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 an image overlapping the second image among the images constituting the first image, and that joins the first image and the second image to generate the synthetic image, using perpendicular bisectors between the 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 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, and wherein the synthetic image generation portion: determines a pixel value of the first image as the pixel value at the predetermined position when the distance is more than a predetermined value and the predetermined position is closer to the first center point than to the second center point, determines a pixel value of the second image as the pixel value at the predetermined position when the distance is more than the predetermined value and the predetermined position is closer to the second center point than to the first center point, and determines a pixel value at the predetermined position by synthesizing the pixel value of the first image and the pixel value of the second image when the distance is not more than the predetermined value.