A three-dimensional image forming system according to the present invention includes a conveyance path along which a heat-expandable sheet is conveyed, a heating unit which heats the heat-expandable sheet by irradiating the heat-expandable sheet with light, a detection sensor which detects that the rear end of the heat-expandable sheet comes close to the heating unit by the conveyance of the heat-expandable sheet along the conveyance path, and a control unit which increases the conveying speed of the heat-expandable sheet whose rear end is detected when the detection sensor detects that the rear end of the heat-expandable sheet comes close to the heating unit.