There is provided a synchronous belt with excellent heat resistance and excellent abrasion resistance, in which a backing rubber is composed of a rubber composition obtained by vulcanizing hydrogenated nitrile rubber with an organic peroxide. At least the lowermost layer of a tooth rubber is composed of a hydrogenated nitrile rubber composition obtained through both sulfur vulcanization and peroxide vulcanization. The coating rubber on a facing fabric is composed of a rubber composition obtained through sulfur vulcanization. The above rubber composition of the tooth rubber is obtained by adding, to every 100 g of hydrogenated nitrile rubber, 0.3 to 10 g of sulfur-containing vulcanizing agent and 0.002 to 0.025 mol of organic peroxide.