A $eta$-glucosidase, efficiently hydrolyzing isoflavone glycoside to isoflavone aglycone, was purified from Pichia guilliermondii K123-1, isolated from Korean soybean paste by ammonium sulfate precipitation, ion exchange column chromatography, gel filtration, and fast protein liquid chromatogram (FPLC). The molecular mass of purified enzyme was estimated to be 45 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDSPAGE). The optimum temperature for enzyme activity was $45^{circ}C$ and it decreased dramatically above $50^{circ}C$. The maximal activity was at pH 4.5 and more than 80% of the activity was retained for 24 hr in the pH range from 4.0 to 8.0 at $4^{circ}C$. The N-terminal amino acid sequence of the enzyme was determined to be GLNWDYDNDK. Based on its substrate specificity and catalytic properties, the activity of the purified $eta$-glucosidase was more effective when the sugar moiety of the glycoside was glucose and the size of the aglycone similar to that of the isoflavones. The purified $eta$-glucosidase efficiently converts genistin and daidzin to genistein and daidzein 1.96 and 1.75 times more than almond meal $eta$-glucosidase.