The value of a diffusing error for an interest pixel is added to image data of the interest pixel, and is calculated from error data occurred in quantization of pixels adjacent to the interest pixel. The image data, to which the value of the diffusing error is added, is quantized using a quantization threshold. Error data of the interest pixel is calculated from the result of the quantization and the image data to which the value of the diffusing error is added. Pixels which are adjacent to the interest pixel and have quantization results corresponding to the image data of the interest pixel are detected, and information between pixels is calculated based on differences between the image data and quantization results of adjacent pixels which fall within ranges corresponding to the detected adjacent pixels. The quantization threshold for the interest pixel is set based on the information between pixels.