A monitoring method and apparatus for detecting an intruding object within a monitoring field of view by applying a subtraction method to an image obtained from an imaging device. A differential image of luminance values between an input image from the imaging device and a reference background image stored in a storage device is generated. The differential image is binarized using a predetermined binarization threshold value and the bianrization threshold value is changed until a noise level of a bianrized image becomes not greater than a predetermined value and based on a binarization threshold value at which the noise level becomes not greater than the predetermined value, an intruding object is detected.