1. An image processing apparatus comprising: background generation means for generating, on each of corresponding pixel positions of more than one input image, a pixel value involved in a predetermined distribution among pixel values of the input images as a background pixel value in the pixel position; degree-of-difference generation means for generating, on each of the pixel positions, a degree-of-difference indicating a degree to which each input image pixel value has a difference from the background pixel value; and output image generation means for generating, on each of the pixel positions, an output image; pixel value by reflecting the input image pixel values depending on the degree-of-difference.