A treating agent comprises a nitrile group-containing highly saturated polymer rubber latex (A) having an iodine value of 120 or less, a rubber latex (B) other than the highly saturated polymer rubber latex, and a water-soluble resorcinol/formaldehyde condensate in amounts of 15 to 80%, 5 to 70% and 2 to 15%, all by weight in terms of solids, respectively. A rubber-reinforcing fiber product is coated with the treating agent. A rubber article such as a toothed belt, a tire and the like is made using this rubber-reinforcing fiber product. The glass fiber product coated with the treating agent may be further coated with a halogen-containing polymer-based adhesive as a secondary treating agent. Thus, the treating agent exhibits a large bonding force between the reinforcing fiber product and a matrix. Even if the rubber article is repeatedly subjected to a flexing stress, the strength thereof cannot be reduced. A peel-off cannot occurs between the reinforcing fiber product and the rubber matrix. Moreover, the rubber article has sufficient heat and water resistances.