In the case where printing density is set to 180 dpi and temperature of a thermal head is under t5, low-density dot-expanding printing is carried out wherein a dot occupying two printing lines for 360 dpi is formed and thermally transferred onto a surface tape by conveying the surface tape and an ink ribbon by two printing lines for 360 dpi in a single printing cycle with heater elements to be used for printing based one line of line printing data being heated.