1. An image generation device comprising: a screen duplication circuit configured to extract and output at least a part of an image area of an input image as a process target image; a mask area determination circuit configured to determine a mask area of the process target image and generate a mask image specifying the mask area; an image quality adjustment circuit configured to perform 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 configured to generate 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.