Unlike in the prior art in which the correspondence between a feature and a judgment value in an intended area is obtained by a single threshold value, the correspondence of the judgment value is obtained independently for each feature by use of a look-up table or the like. This makes it possible to achieve an accurate correspondence between the judgment value and the feature and thus to improve the high processing accuracy. Also, in the prior art, the judgment is repeated several times to secure the accuracy of the judgment and based on the total result thereof, the final judgment is made. Since the accuracy of each process is improved, however, the number of times the judgment is repeated is reduced for a higher processing speed.