An object detecting method, object detecting apparatus and intruding object monitoring apparatus employing the object detecting method. A predetermined monitor area having no object to be detected is imaged by an imaging device and reference background sequential images are registered in advance in a storage unit. When monitoring, the predetermined monitor area is imaged by the imaging device while at the same time sequentially outputting the images frame by frame from the imaging device, a reference background image corresponding to the image from the imaging device is selected from the reference background sequential images, subtraction processing is carried out between the image from the imaging device and a corresponding reference background image, and the object detecting processing is carried out based on the result of subtraction processing.