A thermal printer of the type printing an image on a printing paper wound in the form of a roll. Curling of the printing paper wound in the roll form occurs when the thermal printer is placed in its standby mode over more than a predetermined period of time while the printing paper is held between a platen roller and a thermal head. In order to prevent curling of the printing paper, the printing paper is rewound when the standby mode lasts over more than the predetermined period of time.