The present invention provides a method for producing isoquercitrin, .alpha.-glycosylisoquercitrin, and rhamnose, the method comprising a step of naringin-degrading enzyme treatment during the isoquercitrin production from rutin in the presence of an edible component, such as gelatin, wheat gluten, chitosan, lecithin, a glycerol fatty acid ester, xanthan gum, carrageenan, sodium chondroitin sulfate, casein, enzymatically decomposed gelatin, sodium alginate, konjac extract, gellan gum, guar gum, soybean protein, agar, pectin, yeast extract, egg-white peptide, cluster dextrin, gum arabic, arginine, sodium metaphosphate, karaya gum, locust bean gum, sodium pyrophosphate, glucosamine, chitin, sodium glutamate, dextrin, trehalose, or a grain-based food ingredients. According to the present invention, isoquercitrin and .alpha.-glycosylisoquercitrin, which are of use as antioxidants, anti-fading agents, flavor change inhibitors, etc., can be produced in enhanced yields.