The critical challenge in designing complex digital circuits automatically using an evolutionary algorithm is to overcome the scalability issue caused by combinatorial explosion. Because of this problem, the existing evolutionary design methods can deal with digital circuits composed of only a small number of bits. To resolve this problem, a novel evolutionary design method is proposed on the basis of hierarchical module structures and predominant component prevention. From extensive experiments, it is found that this method can automatically design digital circuits composed of a large number of bits within a polynomial time.