UV-mediated mutagenesis generated a high glucoamylase-producing mutant of Aspergillus oryzae exhibiting strong melanization in solid-state culture. Expression of the glucoamylase-encoding gene (glaB), which is specifically expressed in solid-state culture, and the tyrosinase-encoding gene (melO), was analyzed using an E. coli P-glucuronidase (GUS) reporter assay to investigate this phenomenon. Although no common regulation was found for melO and glaB expression, the former was greatly enhanced in submerged culture. Interestingly, the melO promoter was about four times stronger for GUS production than the powerful promoters amyB, glaA, and modified agdA, previously isolated for industrial heterologous gene expression in A. oryzae. These findings indicated that the melO promoter would be suitable for hyper-production of heterologous protein in Aspergillus. The glaB-type glucoamylase selected as the target protein was produced in a submerged culture of A. oryzae under the control of the melO promoter. The maximum yield was 0.8 g/l broth, and the total extracellular protein purity was 99%. Repeated batch culture, to improve productivity, gave a maximum yield of 3.3 g/l broth. The importance of this work is in the establishment of a both high-level and high-purity protein overproduction system in A. oryzae by use of the melO promoter.