When a color development heat pulse set for each one dot of an intended image is applied to a coloring heat element associated with the one dot of the intended image in a thermal head, an auxiliary heat pulse set for development of the intended image at a thermosensitive medium is applied to a non-printing heat element that adjoins to the coloring heat element associated with the one dot of the intended image with reference to a scanning direction of the thermal head, the auxiliary heat pulse being not so hot for color development at the color thermo sensitive medium.