A three-dimensional display device includes: a display region candidate deciding unit that decides a candidate region of an additional image which shields part of a main image of a three-dimensional image on a screen; a depth suitability determination unit that determines whether a difference in depth between the main image and the additional image is within a tolerance range; an image compositing unit that superimposes the additional image upon the candidate region on the main image, and displays the composited image; and a possibly-unsuitable region deciding unit that decides, in the main image, a first region that may protrude to a near side beyond a predetermined depth range, and a second region that may recess to a far side beyond a predetermined depth range. The display region candidate deciding unit further decides a candidate region to shield the first and second regions.