An image processing apparatus according to the present invention comprises an input unit sequentially scanning first image data having first resolution, in units of a predetermined number of pixels that are contiguous in a first direction, in a second direction crossing the first direction at right angles, thereby inputting a pixel group corresponding to each unit of pixels, a computation unit calculating pixel values of second image data having second resolution using pixels in a reference area having a predetermined range in the first image data, a storage unit storing pixel values of a range of the first image data that is larger than the reference area in the first direction, and a control unit controlling the computation unit so as to successively output pixels of the second resolution in the first direction in units of the predetermined number of pixels using the stored pixels.