1. An object detecting device for detecting if a given grayscale image is an object, the device comprising: a plurality of weak discriminating means for computing an estimate indicating that the grayscale image is an object or not according to a characteristic quantity that is equal to the difference of the luminance values of two pixels at two different positions; and a discriminating means for judging if the grayscale image is an object according to the estimate computed by one of or the estimates computed by more than one of the plurality of weak discriminating means.