Aspergillus usamil D5, which produces ${eta}$-glucosidase to hydrolyze isoflavone glycosides of soybean and Astragali Radix into their aglycones efficiently, was isolated from Korean traditional koji, Nuruk. ${eta}$-Glucosidase produced by the isolated A. usamii D5 was purified to homogeneity through 80% ammonium sulfate precipitation, DEAE Sephadex A-50 column chromatography, Sephacryl S-200 HR column chromatography, and FPLC equipped with Mono Q and Superose columns. The molecular mass of the purified ${eta}$-glucosidase was estimated to be about 128 kDa. The purified ${eta}$-glucosidase was stable between 20 and $45^{circ}C$ but unstable above $45^{circ}C$. The optimum temperature of the enzyme was $55^{circ}C$ at pH 4.5. The purified ${eta}$-glucosidase was stable over a broad pH range from 3.0 to 6.5, but its stability rapidly decreased at pH over 6.5. Maximal activity was observed at pH 4.5 in 100 mM sodium citrate buffer. The enzyme activity was inhibited to 13.2, 17.8, and 5.0% by the addition of $Cu^{2+}$, $Ag^+$, and $Hg^{2+}$ and was reduced to 3.7 and 40.4% by the addition of glucose and maltose, respectively. The purified ${eta}$-glucosidase showed the highest level of activity to o-nitrophenyl-${eta}$-D-glucoside. The purified ${eta}$-glucosidase efficiently hydrolyzed isoflavone glycosides such as daidzin and genistin extracted from soybean and formononetin and calycosin extracted from Astragali Radix to their aglycone forms.