An intruding object detecting method and an intruding object monitoring apparatus for detecting a target object intruding into an image pickup region while reducing the error detection of moving objects other than the target object. A difference in pixel value between an input image signal and each of different image signals in a predetermined number of frames greater than one is calculated for each pixel to thereby obtain differential images in the predetermined number of frames. The differential images in the predetermined number of frames are synthesized in predetermined proportions to thereby generate a synthesized differential image. The synthesized differential image is binarized on the basis of a predetermined threshold value to thereby generate a binarized image. A binarized object in the binarized image is detected as an object intruding into a monitoring visual field.