A three-dimensional image forming system includes a high congestion area specifying unit that specifies, in print data for printing a grayscale image used to thermally expand a desired area on a thermally expandable sheet, a high congestion area which is an area having higher congestion of a density pattern included in the grayscale image than a predetermined value and a density adjusting unit that adjusts the density of the grayscale image such that the expanding height of the high congestion area is to be a desired height.