Quantizing means generates a quantized density and a quantization error component for a target pixel in a plurality of pixels. A subtracter calculates, for the target pixel, a quantization error correction component by subtracting the density of the target pixel contained in preliminary ejection data from the quantization error component output from the quantizing means. An allocator allocates the quantization error correction component calculated by the subtracter to a plurality of the pixels around the target pixel. The quantizing means outputs the quantized density and the quantization error component of the target pixel by performing an addition using the density of the target pixel contained in the image data, the density of the target pixel contained in the preliminary ejection data, and the accumulated diffused error component assigned to the target pixel by the allocator when other pixels are quantized.