PURPOSE: To contrive to conserve resources and prevent environmental pollution due to unrequired documents, by a method wherein characters or symbols are printed on a thermo-sheet by a thermal head, and unrequired printed thermo- sheets are exposed to a low-temperature atmosphere to delete the printed images.

CONSTITUTION: A base sheet 2 consisting of a polyester film or the like is wound around a reel 1, and a heat-sensitive material 4 exhibiting reversibility in a low-temperature atmosphere, e.g., Ag2HgI4, is applied thereto. A protective sheet 6 consisting of a transparent polyester film or the like is laminated on the base sheet 2 by a laminator 7, and the laminate is cut to size by a cutter 9 to obtain thermo-sheets 10. The heat-sensitive material develops a color when being heated to a temperature of not lower than 80°C, and exhibits reversibility in a temperature region of not higher than 5°C, whereby the developed color maintained in a higher temperature region is removed. Since the color-developed condition is maintained in an atmosphere of a temperature of higher than 5°C, the printed images will not disappearin an ordinary room condition.