We propose an active learning method with hidden-unit reduction. which is devised specially for multilayer perceptrons (MLP). First, we review our active learning method, and point out that many Fisher-information-based methods applied to MLP have a critical problem: the information matrix may be singular. To solve this problem, we derive the singularity condition of an information ma(cid:173) trix, and propose an active learning technique that is applicable to MLP. Its effectiveness is verified through experiments.