An image processing apparatus sets a variable-magnification ratio of image data, discriminates whether or not the set variable-magnification ratio is larger than a predetermined threshold value, and decides a memory size of a memory device for storing the image data in accordance with a discrimination result in the discriminating. The Apparatus stores the image data of an amount corresponding to the decided memory size into the memory device, and executes a variable-magnification process to the image data stored in the memory device by using the set variable-magnification ratio.