Aspergillus oryzae producing high proteolytic enzyme was isolated from soybean koji and named tentatively A. oryzae O-1. A. oryzae U-1 was obtained by mutation of A. oryzae O-1 with ultraviolet (UV)-irradiation and produced 14 times higher pretense activity compared with A. oryzae O-1. A. oryzae E-1 was acquired by treatment of A. oryzae U-1 with 0.5 M ethylmethanesulfonate (EMS) for 6 min at 3$0^{circ}C$ and produced 39 times higher proteolytic activity than A. oryzae O-1. With protoplast fusion between A. oryzae O-1 and A. oryzee E-1 in the presence of polyethylenegylcol (PEG)-CaCl$_2$, proteolytic activity was increased to 82 times compared to A. oryzae O-1, and the fusant was named A. oryzae PF. The activities of the cultures containing proteolytic enzymes produced by the strains were determined to be 0.23 U/$mell$ for A. oryzae O-1, 3.29 U/$mell$ for A. oryzae U-1, 8.91 U/$mell$ for A. oryzae E-1, and 19.0 U/$mell$ for A. oryzae PF.