An object such as a person is extracted from images captured by cameras (2R, 2L) to determine the degree of vertical symmetry of the object (step 33). In the case where the degree of vertical symmetry is determined to be high, the object is determined to be other than a living body (step 33a). In the case where the degree of vertical symmetry is determined to be low, the object is determined likely to be a living body (step 33b). In the case where determined likely to be a living body, the object is further determined to be a predetermined kind of living body (for example, a person) or not based on the image of the object (steps 34 and 35). This allows an extremely reliable and simple determination of whether the type of the object extracted from the image captured by the camera is a living body such as a person.