To reduce a color shift by introducing a random element to error diffusion processing.

An image input terminal equipment 12 sends a signal to an input RAM 70. Then a corrected image value (In+En-1) is transferred to a threshold comparator 76. On the other hand, a threshold correction element (k-1) corresponding to an input level In is stored in a lookup table 78 and selected depending on the input level In. Then a value In(k-1) is subtracted from a threshold To at a threshold calculation subtractor 80 to decide a threshold Tn to be added by the threshold comparator 76. Then an error total section 49 is accessed and the threshold Tn is corrected. Then the threshold comparator 76 compares the corrected image value (In+En-1) with the corrected threshold Tn to decide a proper signal level Bn.