An image processing apparatus which processes input image data of Y lines, each consisting of X pixels, using an SIMD processor, comprises a calculation unit including N (X>N>1, Y>N>1) elemental processors capable of parallel-operating; an input unit for dividing and inputting the image data of one line with respect to every N pixels; a storage for storing the input N-pixel image data of the N lines; and an image processor for supplying, from among the stored N-pixel image data of the N lines, the N image data respectively to the N elemental processors, and causing the respective elemental processors to perform the same-kind calculations in parallel. Thus, the image processing apparatus for performing an image process such as error diffusion by using the SIMD processor without using any auxiliary processor for a sequential process can be provided.