A three-dimensional image data generation system includes: an editing unit configured to edit grayscale image data in which a density level for specifying an foaming height of a thermally expandable sheet is set for each coordinate in a planar direction; and a conversion unit configured to, when the editing unit changes a size in the planar direction of an image region included in the grayscale image data, convert the density level in correspondence with a ratio of the image region between before and after the change.