1. An apparatus for classifying pixels in each frame of a motion picture as foreground or background comprising: distribution generating means for generating a distribution value for a pixel representing an occurrence probability of a pixel value of the pixel based on pixel values of pixels located at the same position in each frame as the pixel; decision means for classifying a pixel as a foreground pixel, a background pixel or an undefined pixel based on a first threshold, a second threshold and the distribution value of the pixel; and adjustment means for sorting each pixel into a foreground pixel or a background pixel based on the classification result of the decision means.