A flavonoid solubilization agent capable of highly solubilizing flavonoids such as isoflavone, baicalin, rutin and naringin whose solubility is generally low; and a method of effecting the above solubilization. Flavonoids can be solubilized by causing a flavonoid and soybean saponin and/or malonyl isoflavone glycoside to be co-present in an aqueous medium.