It is made possible to provide a three-dimensional image display device by which an alarm image with high visibility can be displayed at the end of the viewing zone in a parallel-ray one-dimensional IP system, without a reduction of the viewing zone or 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 elemental images; an optical plate that is placed to face the elemental image display unit, has optical apertures that extend linearly in a vertical direction and are arranged at regular intervals in a horizontal direction, and controls light rays from the elemental image display unit; and an image data converting unit that converts image data so that single-color portions having periodically varying widths are inserted to boundary portions between the elemental images, the width varying with locations in the elemental image display unit, the inserted single-color portions being asymmetrical in the entire elemental image display unit.