When the pixel of interest exists at a pixel position where reference of an error buffer is necessary, a quantization error generated in a region having undergone error diffusion processing is obtained from the error buffer. A quantization error generated in error diffusion processing in a region containing the pixel of interest is obtained from an error memory. Diffusion errors are calculated from the quantization errors using an error diffusion matrix. The value of the pixel of interest and the diffusion errors are added, and the addition value is quantized into the quantization value of the pixel of interest. The difference between the quantization value and the addition value is calculated as a quantization error to be stored in the error memory. When the pixel of interest exists at a pixel position where write in the error buffer is necessary, the quantization error is stored in the error buffer.