An apparatus for forming a three-dimensional image includes: a print function unit that forms a first print image on one surface and a second print image on a region of another surface of a thermally expandable sheet with a photothermal conversion material; and an image processing unit that sets a density of the photothermal conversion material in the second print image based on a density of the photothermal conversion material in the first print image. The second print image is a mirror image of the first print image, and the region of the other surface corresponds to the first print image of the one surface. A three-dimensional image of the first print image is formed by selectively expanding a thermally expandable layer with thermal energy according to the density of the photothermal conversion material in the second print image when the sheet is irradiated with light from the other surface.