PURPOSE: To obtain new water-soluble substance Af90-NF effective as a dental caries preventing agent, having glycosyltransferase inhibitory activity.

CONSTITUTION: New substance Af90-NF having physical and chemical properties of a brown cotton shape, not showing a melting point, decomposing at ≥270°C, having an elemental analysis value of C 42.5%, H 3.1%, N 0% ash 5.1%, 1,060 weight-average molecular weight using N,N'-dimethylformamide (0.05N LiCl, 0.05 M phosphoric acid) as a mobile phase based on polyethylene glycol as molecular weight standard by gel permeation chromatography, having infrared absorption spectrum and ultraviolet absorption spectrum shown by the table, color reaction positive in ferric chloride reaction and negative in aniline and diphenylamine reaction, etc., solubility: insoluble in hexane, slightly soluble in methanol, soluble in water. The compound is obtained by extracting fruits of Alnus firma with hot water, can prevent glucan to cause bacterial plaque from forming and has not side effects at all.