1. A sample class prediction method comprising: a) obtaining a discriminant function for performing classification/prediction to assign each training sample in a training sample set to class 1 or class 2 on the basis of a plurality of explanatory variables generated for said each training sample; b) calculating a discriminant score for said each training sample and a discriminant score for an unknown sample by using said discriminant function; c) determining whether the discriminant score of said unknown sample is either not smaller than the largest discriminant score or not larger than the smallest discriminant score taken among all of said training samples; d) if the determination in said c) is affirmative, then determining the class to which said unknown sample belongs in accordance with the discriminant score of said unknown sample; e) if the determination in said c) is negative, then removing at least the training sample having said largest discriminant score and the training sample having said smallest discriminant score from said training sample set, and constructing a new training sample set by using remaining training samples; and f) repeating said a) to e) for said new training sample set.