1. A motion detection apparatus for detecting motion of an inputted image, comprising: reference image storage means for storing the image as a reference image to be used as a reference for detection of motion; updating means for updating the reference image stored in said reference image storage means, said updating means being operable in at least three alternative updating modes: an unconditional updating mode in which the reference image is periodically updated, a conditional updating mode in which the reference image is periodically updated until motion is detected whereupon updating of the reference image is inhibited, and an updating inhibition mode in which the reference image is not updated; current image storage means for storing an image inputted now as a current image; motion detection means for comparing the current image stored in said current image storage means with the reference image stored in said reference image storage means and detecting motion of the image based on a result of the comparison; and extension means for extending an interval within which motion of the image is detected by said motion detection means by a predetermined extension time.