An image processing apparatus having general versatility, for flexible quantization on multivalue image data of two or more colors to image data having a reduced number of bits in correspondence with the number of bits to be quantized, by using an error diffusion scheme. When two color multivalue data are inputted, errors distributed to the position are added to the data and inputted as I'.sub.0 and I'.sub.1 into an output determination unit 2. The input data I'.sub.0 and I'.sub.1 are quantized by using LUTs 7-0 and 7-1 into numerical values of less-value representation. The two data from the LUTs 7-0 and 7-1 are inputted into an LUT 4, as coordinate values in a two-dimensional coordinate system, and two color binarized bit data (total 2 bits) are outputted.