An image processing apparatus comprises a processor for quantizing multi-gradation image data with regard to each of R, G and B to convert it into pseudo halftone output image data. First, the processor performs a first quantization for deciding a dot appearance pattern C(B)i,j[k] with respect to an observed pixel of the multi-gradation image data with regard to blue. After that, the processor performs a second quantization for deciding the dot appearance pattern of green with respect to the observed pixel so as to make an anti-correlation with the dot appearance pattern C(B)i,j[k] decided by the first quantization.