A dental plaque-degrading composition containing endodextranase produced by Arthrobacter globiformis W31 exhibiting high degradation efficacy to insoluble glucan produced by Streptococcus mutans IFO 13955, the dental plaque-degrading composition additionally containing .alpha.-amylase of various origins. These compositions are useful for preventing the settlement of Streptococcus mutans in the oral cavity and the formation of new dental plaque and for preventing the dental caries, and provided in the form of dentifrices, denture detergents, troches, mouth washes, chewing gums or candies.