PURPOSE: To provide the subject safe inhibiting agent containing a tree- originated tannin as an active ingredient, having a strong glucosyl transferase- inhibiting action, preventing the formation of bacterial plaques or the generation of dental caries, and useful as a dental caries-resistant agent for oral cavity hygienic agents or food additive agents.

CONSTITUTION: This inhibiting agent contains as an active ingredient a condensed tannin originated from the core or bark of a tree, e.g. a condensed tannin having a weight-average mol.wt. of 800-10,000 and obtained by extracting the core or bark of a tree belonging to the Anacardaceae, the Pinaceae or the Leguminosae, especially acacia bark or the bark of Larix leptolepis, with a solvent such as methanol, ethanol, ethyl acetate or (hot) water at a temperature of 15-35°C under shading and subsequently purifying the obtained extract with an adsorption column chromatography.