To suppress the occurrence of composite black in a low gray level area, to improve the picture quality and to speed up the processing by holding the binarized result of a first color component as information in a noticed picture element and setting the output level of a second color component to be zero when the binarized result of the first color component is 'on'.

A judgment operation of first color component data of a noticed picture element by a first color component ON/OFF judgement part 103 is omitted. An error weighting computing unit 104 weights error data. The weighted data are binarized based on a binary threshold by a binarization processing part 105. The binarized result of the first color component is stored in a first color component ON/OFF data storage part 108. The first color component ON/OFF judgment part 103 refers to the first color component ON/OFF data in the noticed picture element and when the first color component is 'ON', the output level of second color component data is set to be zero.