1. An image generation device comprising: a screen duplication circuit that extracts at least a portion of an image area of an input image as a process target image and outputs a composite duplicate image in which at least three of the process target images are arranged next to each other; a mask area determination circuit that determines a mask area of the process target image and generates a mask image specifying the mask area; an image quality adjustment circuit that performs a predetermined image process to the process target image in the mask area determined by the mask area determination circuit and output an image to which the image process is performed as an image after image quality adjustment; and a composition circuit that generates a composite image in which the process target image, the mask image, and the image after the image quality adjustment are composited such that the process target image, the mask image, and the image after the image quality adjustment are displayed on a screen by replacing one of the process target images in the composite duplicate image with the mask image and replacing another one of the process target images in the composite duplicate image with the image after the image quality adjustment; wherein the composition circuit composites the process target image, the mask image, and the image after the image quality adjustment such that the mask image is displayed between the process target image and the image after the image quality adjustment.