Problem to be solved. To provide methods for improving bioavailability of quercetin and a method for preventing reduction of quercetin solubility due to substances that insolubilize quercetin, and compositions containing quercetin with improved bioavailability. Solution. The invention provides a method for improving bioavailability of quercetin by using a quercetin glycoside, and a method for inhibiting the solubility reduction of quercetin with a substance that insolubilized quercetin. The invention also provides a quercetin-containing composition characterized by the ratio of quercetin-insolubilizing substance to quercetin glycoside (quercetin-insolubilizing substance/quercetin glycoside) is 1 or higher.