An image processing apparatus comprises a pixel group reference point determination unit 64 that divides the input image data into predetermined pixel groups and determines a position of a reference point from the grayscale values of respective pixels contained in the pixel groups; a pixel group expansion processing unit 65 that takes grayscale values of the part that is less than a threshold value from unprocessed pixels in the periphery of the pixel group to the pixel group if the sum total of the grayscale values of the respective pixels contained in the pixel group is less than the threshold value; and a quantization data supply unit 66 that supplies the quantization data to a pixel on the basis of the position of the reference point determined by the pixel group reference point determination unit 64.