1. An image processing apparatus including a computer, comprising: a background generator 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; a degree-of-difference generator 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 a locus synthesizer 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, wherein the background generator includes a frequency-of-occurrence holder for holding a frequency-of-occurrence of the background pixel value.