PURPOSE: To obtaian a good printing result leaving no gap between lines by dividing the pulse current group of the number of times corresponding to gradation density into a plurality of groups in a thermal printer and supplying a current to heating resistors in a dispersed state during the moving period of one line of recording paper.

CONSTITUTION: The number of a pixel is set to (i) and, at first, (i) is set to 0. Next, the density gradation data of the i-th pixel is inputted from a buffer 202 and compared with comparison data (k) as (di) and, when (di) is equal to or less than (k), bit row data (bi) is set to 0 and, when (di) is other than (k), (bi) is set to 1. (bi) is outputted to an one bit shift register 205. The pixel number is made incremental by 1 to confirm whether the processing to all of the pixels corresponding to one line is completed and, when (bi) is judged to be sent to the shift register 205, one strobe signal 209 is outputted to perform the printing corresponding to one density gradation. The comparison data (k) is increased by 2 and comparison processing is repeated until (k) becomes 15 and 1 is substituted for the comparison data (k) to repeat the same processing.