A novel isoflavone glycoside-hydrolyzing beta-glucosidase produced by Lactobacillus casei subsp. rhamnosus IFO 3425 was solubilized by ultrasonic disruption of the cells in the presence of 2-mercaptoethanol and sorbitol as stabilizer. The beta-glucosidase from L. casei subsp. rhamnosus specifically hydrolyzed soybean isoflavone glycosides, namely, daidzin and genistin, converting them to daidzein and genistein, respectively. By contrast, a commercial preparation of almond emulsin beta-glucosidase could not hydrolyze these soybean isoflavone glycosides. The undesirably bitter and astringent isoflavone glycosides in soybean were decomposed for the first time with this novel beta-glucosidase, an enzyme which has hitherto been considered difficult to solubilize, produced by a lactic acid bacterium.