An error diffusion method includes: simultaneously receiving first to nth (n is a positive integer of 2 or larger) pixel data at every clock; adding a quantization error stored in a memory to each of the first to (n−1)th pixel data and quantizing them into data having a smaller number of bits than the number of input bits; adding the quantization error stored in the memory to the nth pixel data and quantizing it into data having a smaller number of bits than the number of input bits; diffusing the quantization errors of the first to (n−1)th pixel data to nearby pixels excluding the first to nth pixels by using a first error diffusion mask, and storing the diffusion results of the quantization errors of the first to (n−1)th pixel data in the memory; and diffusing the quantization error of the nth pixel data to pixels around the nth pixel by using a second error diffusion mask, and storing the diffusion results of the quantization error of the nth pixel data in the memory.