11 5 11 7 11 4n 3 11 4n 1 as is when the strobe pulse signal becomes off at timing t3 after elapse of a given period of time the switching elements 12 1 12 3 12 5 12 7 12 4n 3 12 4n 1 are correspondingly brought out of conduction completing application of a voltage to the heating elements 11 of the first and third groups in fig2 for brevity the strobe pulse width is set to the maximum width corresponding to the dots having an area ratio of 100 as a matter of course the timing at which the strobe pulse signal is completed i e the width of the strobe pulse signal changes according to the input print data so as to correspond to the respective heating element 11 subsequently the shift register circuit 14 waits for another new print data set and voltage application is repeated after timing t4 in a similar manner described previously fig3 a shows a dot pattern of black k thus formed numerals 1 though 4 provided in the drawing represent element numbers corresponding to respective dots in the drawing the primary scanning direction designates the direction in which the heating elements are positioned and the secondary scanning direction designates the direction in which paper is fed fig3 b 3 c and 3 d show dot patterns of the other colors c m y when the start timing of heating of each group is controlled in accordance with the combinations provided in fig1 according to the foregoing embodiment as is evident from a comparison between the present invention shown in fig2 and the conventional one shown.