The recording sheet is virtually divided into a recording area and an unrecording area in a main scan direction. Unrecording area printing heat energy is Esi calculated by subtracting printing heat energy Epi for a line to be printed from maximum printing heat energy Epmax for printing each line in the recording area. Common printing heat energy is calculated by dividing the unrecording area printing heat energy Esi by the number of the heating elements in the unrecording area. Virtual image data for each heating element is obtained by counting backward from the common printing heat energy. A recording sheet is printed based on the virtual image data and the real image data. Total printing heat energy is kept constant in each line, and there occurs no printing heat energy fluctuation on printing. Transport load fluctuation due to the printing heat energy fluctuation is suppressed to prevent an uneven density.