The present invention provides an image-processing apparatus, including: a corrected value calculating unit configured to calculate corrected values by adding error values diffused from a pixel neighboring a pixel currently being processed respectively to a plurality of input values corresponding to the pixel currently being processed; a total value calculating unit configured to calculate a total value of all corrected values; a quantizing unit configured to quantize the total value; a distributing unit configured to calculate a plurality of output values corresponding to the plurality of input values by distributing the quantized value according to a ratio between each of the corrected values and the total value; and an adding unit configured to calculate respective difference values between the plurality of output values and the plurality of corrected values, and to add the calculated difference values to corrected values.