When a print line is printed on a printing paper by supplying a print current to a thermal head, heating elements within the thermal head and a transfer ribbon are easily adhered together due to melted ink on surface of the transfer ribbon. Due to such adhering between the heating elements and the transfer ribbon, a white line is easily formed on a printing paper. In order to prevent such white line from being formed, the heating elements must be prevented from being cooled down after printing each print line. More specifically, in a period between a first time when each print line is completely printed and a second time when a printing paper is to be transported forward to a next print line, the heating elements are heated by relatively small heating value which is sufficient to melt a adhering portion formed between the heating elements and the transfer ribbon but which is insufficient to perform the printing. Thus, the heating elements are prevented from being adhered to the transfer ribbon, so that each print line can be printed with accuracy and without forming the white line.