1. An image processing apparatus for quantizing input multivalued image data by a multivalued error diffusion method, selecting a predetermined dot pattern based on the quantized image data and outputting a binary image, comprising: error calculation means for calculating corrected value from a gray-level pixel value and a processed pixel diffusion error of the input image, and calculating a quantization error from said pixel value and an output density level corresponding to the corrected value; image generation means for first acquiring a diffusion coefficient corresponding to a pixel value of said input image, and distributing said quantization error to surrounding pixels according to a weight assignment by the diffusion coefficient to generate a binary image; diffusion coefficient generation means for generating a plurality of candidate diffusion coefficients; computing means for acquiring an evaluation value for the binary image generated by said image generation means; and selection means for selecting the diffusion coefficient corresponding to the pixel value of said input image from said plurality of candidate diffusion coefficients based on said evaluation value.