PURPOSE: To obtain a recording image of high quality with high resolution by printing two succeeding lines divisionally plural times when the black rate of a one line exceeds such a predetermined black rate that sticking is prevented.

CONSTITUTION: An image signal read by an image signal reading circuit 1 is inputted to a thermal head driving circuit 2 and a black rate detecting circuit 5 successively in series. The black rate detecting circuit 5 counts the amount of the black image signal of the image signal inputted in series, line by line, to calculate the black rate N of every time and apply to a comparator 6. The black rate N0 (=50%) below which no sticking occurs is set in a black rate setting circuit 4, and this is compared with the black rate N detected by the comparator 6. When the detected black rate N exceeds the rate N0, 50% of the image signal of the corresponding line is printed and the remaining 50% is printed after recording paper is fed reversely by one line.