It is made possible to provide a three-dimensional image display device by which a viewing zone can be readily adjusted in the parallel-ray one-dimensional IP system, without a decrease in processing speed. A three-dimensional image display device includes: an elemental image display unit that has pixels arranged in a matrix form in a display plane, and displays an elemental image; an optical plate that is placed to face the elemental image display unit, extends straightly in a vertical direction, and controls a light ray from the elemental image display unit, the optical plate having optical apertures arranged at intervals in a horizontal direction; and an image data converting unit that converts image data so as to replace missing image data with a neighboring parallax component in same the elemental image, based on the average width of elemental images in an input image data and the optimum average width of elemental images in a three-dimensional video image to be output.