A control section 9 of a thermal printer 1 performs CMYK/RGB-gray conversion, gradation conversion, halftone conversion for an input image data 3, and performs shift processing and rotation processing in accordance with the resolution and the number of lines of the thermal printer 1. The control section 9 performs heat-accumulation correction processing for the image data subjected to the halftone processing, and the image data subjected to the shift processing and the rotation processing, and then prints a hairline image. When a circular hairline is printed, the control section 9 defines a pattern of circular hairlines, and produces a horizontal hairline pattern. Further, the control section 9 shifts the produced horizontal hairline pattern downward to thereby produce another horizontal hairline pattern, performs polar coordinate conversion for the produced horizontal hairline patterns to thereby produce two circular hairlines, performs the heat-accumulation correction processing for the two produced circular hairlines, and prints them.