1. An image processing method utilizing an error diffusion technique, comprising the steps of: (a) distributing binary-coding errors, generated when multivalued density data of respective pixels of an image are converted into binary-coded data, to peripheral pixels having a predetermined positional relation with respect to the respective pixels; (b) calculating an error sum by adding up binary-coding errors distributed to an object pixel from peripheral pixels around the object pixel; (c) comparing an addition result obtained by adding a density value of the object pixel to the error sum calculated for the object pixel with a predetermined binary-coding threshold value, and obtaining binary-coded density data of the object pixel based on a comparison result; and (d) comparing the addition result obtained by adding the density value of the object pixel to the error sum calculated for the object pixel with predetermined reference values which can be variably set, and calculating a binary-coding error for the object pixel.