The image data including a grid pattern to be printed by a thermal printer line by line is stored for three lines of dots in a plural line buffer, and the image data is scanned by a window frame of an inverted T-shape which covers the three lines. When a dot arrangement extracted by the window frame including an object dot and its surrounding dots coincides with a predetermined window frame pattern defined in an intermediate table, an address representing the dot arrangement is convertred into an intermediate code by the intermediate table. The intermediate code indicates the amount of heating energy to be supplied to a heating element corresponding to the object dot in order to preheat when the area of the object dot is a non-printing area, or to heat additionally when this area is a printing area, thereby to prevent a thin or broken portion from appearing in the printed pattern line.