A toothed belt is provided that has excellent environment resistance, high load resistance, high stiffness, and achieves high accuracy, light weight, compactness and low noise.A toothed belt is provided with a belt body including a plurality of core wires juxtaposed in the width direction of a rubber portion, a plurality of tooth rubber portions formed on one surface of the belt body, and a tooth cloth covering the surface of the tooth rubber portions. The rubber portion contains dioctyl sebacate and trimellitic acid ester in a mass part ratio of 1:19 to 1:1, the mass percentage of the dioctyl sebacate to the whole quantity of the rubber portion is less than 4 mass %, and the mass percentage of the total quantity of the dioctyl sebacate and the trimellitic acid ester to the whole quantity of the rubber portion is in a range from 4 mass % to 8 mass %.