1. An image processing apparatus to detect a mixed area from image data which is formed of a predetermined number of pieces of pixel data obtained by an image-capturing device including a predetermined number of pixels, the pixels having a time integrating function, the mixed area being obtained as the pixel data in which a plurality of objects are mixed in the real world, said image processing apparatus comprising: a motion compensation unit configured to compensate for the motion of frames of the image data; and an area detector configured to subtract pixel data at a corresponding position in the motion-compensated frames, determine a difference between the subtracted pixel data, and detect the mixed area based on the difference between the pixel data at the corresponding position in the motion-compensated frames, wherein said area detector detects the mixed area to which the pixel data belongs when the difference is greater than or equal to a threshold, wherein a shutter-time period is divided into a plurality of equal periods, each pixel data being divided in accordance with the plurality of periods and pixel data corresponding to each of the plurality of equal periods being obtained for estimating a mixed ratio of each pixel in the mixed area, and wherein the motion compensation unit generates the motion-compensated frames by matching backgrounds of the image data.