1. An object detecting device for detecting if a given gradation image is an object or not, the device comprising: a plurality of weak discriminating means for computing an estimate indicating that the gradation 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 that is learnt in advance; and a discriminating means for judging if the gradation image is an object or not according to the estimate computed by one of or the estimates computed by more than one of the plurality of weak discriminating means.