The image processing method converts an input image having multiple tones into an image having fewer tones than the input image, for image recording by a recording head including a plurality of recording elements arranged in a prescribed direction, the method comprising the steps of: quantizing pixel values of pixels of the input image in a processing direction which is set to a direction substantially parallel to a relative conveyance direction of a recording medium with respect to the recording elements when recording onto the recording medium is performed by means of the recording elements, the pixel value for a pixel under processing being quantized according to a result of a comparison between the pixel value for the pixel under processing and a threshold value for the pixel under processing; diffusing an error with respect to the pixel under processing caused by quantizing the pixel value for the pixel under processing, into an unprocessed pixel in a periphery of the pixel under processing; and determining a threshold value for the unprocessed pixel of the input image in accordance with the error.