A three-dimensional display device configured to display a main image and an additional image on a screen includes a display region candidate decider that decides one candidate region from a plurality of region candidates for the additional image to be superimposed on the main image on the screen, a depth suitability determiner that determines whether a difference between a depth of the main image displayed at a boundary region and a depth of the additional image is within a predetermined tolerance range, and an image composer that, when the difference in depth between the depth of the main image displayed at the boundary region and the depth of the additional image is within the tolerance range, superimposes the additional image upon the main image at the candidate region, thereby composing a composite image of the main image and the additional image, and displays the composite image on the screen.