an object of this invention is to prevent an increase in image data while preventing degradation of the image quality caused by an image process in a digital image processing system . to achieve this object , a signal processing apparatus which quantizes pixel values within a pixel area having m pixels into data of 2 n steps by n bits expresses a decimal part of data in the entire pixel area by increasing or decreasing by one a predetermined number of pixel values among the m pixels on the basis of the decimal part of the data in the entire pixel area .