Each picture element is made to correspond to a plurality of dots and the ratios of printing energy corresponding to respective dots in each picture element are determined to correspond to respective tones. The heat accumulation state of a heat generating member corresponding to a picture element to be printed is detected from the tones of picture elements about the picture element to be printed for controlling the printing energy supplied to the heat generating members corresponding to respective dots of the picture image to be printed in accordance with the printing energy ratio and the detected state. With this method it is possible to provide a half tone print having uniform high quality regardless of the tone of a previous line and tones of surrounding picture elements.