In an image encoding device, a predicted pixel generator generates a predicted value for pixel data based on at least one of pixels adjacent to a pixel to be compressed. A quantization processor quantizes a difference value between the pixel data and the generated predicted value to a quantized value having a smaller number of bits than that of the pixel data to compress the pixel data to the quantized value. In a quantization width determiner, an edge determiner determines whether or not there are both a flat portion and an edge portion, based on a characteristic of the difference value in the group, and an edge pixel determiner determines whether each of pixels included in the group is a pixel in an edge portion or a pixel in a flat portion, based on the difference value, whereby a quantization width used in the quantization processor is determined.