A simple and easy process for preparing pharmacologically useful flavonoid compound having reductase inhibitory effect, active oxygen extinguishing effect, carcinogenesis promotion inhibitory effect, anti-inflammatory effect, and so on. Particularly, a process for preparing the compound of the formula (I): wherein, R2 is a substituted or un-substituted phenyl group; R7 is a hydrogen atom or a hydroxyl group; and n is an integer of 1 to 4; by bonding a sugar derivative to catechins as the starting compound selectively via O-glycoside bond and then oxidizing the 4-position of flavanoid skeleton of the obtained compound.