A photothermal conversion image generating device that generates a photothermal conversion image for forming a three-dimensional image by causing a thermally expandable sheet having a thermally expandable layer on a base material to expand, the device including: a processor that allocates, based on a line width of a line forming the three-dimensional image, an image for forming the line to either a front surface of the thermally expandable sheet or a rear surface of the thermally expandable sheet.