In a thermal printing utilizing a heat sensitive printing paper or an ink donor film, sticking of a heat sensitive color developing layer on the printing paper or a substrate film of the ink donor film to a thermal head is prevented by supplying a predetermined amount of electric energy to heat generating elements of the thermal head during time intervals between successive printing heat generations in the heat generating elements. For this purpose, warming pulses generated from a warming pulse generating circuit are applied during the time intervals to the heat generating elements for warming the same.