1. A control device that displays, on a display, a plurality of diagnosis images each diagnosis image being a combination of a natural image and a pattern image, wherein the control device including an output controller configured to display, on the display, a first diagnosis image being a combination of a natural image and a pattern image, and display a second diagnosis image also being a combination of a natural image and a pattern image and in which a portion of the natural image and the pattern image are similar, compared with the first diagnosis image, after displaying the first diagnosis image, wherein the output controller is further configured to display, on the display, a third diagnosis image having the same pattern as the first diagnosis image but which is different from the first diagnosis image in at least a position of the pattern image, and in which the portion of the natural image and the pattern image are not similar in the third diagnosis image, compared with the second diagnosis image, after displaying the second diagnosis image, and display, on the display, a fourth diagnosis image having the same pattern as the second diagnosis image but which is different from the second diagnosis image in at least the position of the pattern image, and in which the portion of the natural image and the pattern image are similar in the fourth diagnosis image, compared with the third diagnosis image, after displaying the third diagnosis image.