When printing is performed using a thermal head to convert power into heat and to heat an ink sheet laid on paper by the heat, a change in density in a paper carrying direction D1 of an image to be printed in an output region of the paper is calculated using printing data, printing-data-for-correction to indicate an image-for-correction to be printed in a margin printing region of the paper and to cause a change of power required by the thermal head while a combined image including the image and the image-for-correction is printed onto the paper to be smaller than a change of the power required by the thermal head while only the image is printed onto the paper is created, and the ink sheet is heated by the thermal head in accordance with the printing data and the printing-data-for-correction.