The present invention relates to a toothed belt containing tooth parts that are disposed at predetermined intervals along a belt longitudinal direction, a cord, and a back side that has the cord embedded therein, in which the cord contains a Lang-twisted glass fiber, an adhesion treatment layer, and an overcoat treatment layer, the overcoat treatment layer is formed on a surface of the adhesion treatment layer, the back side is composed of a rubber composition, and the rubber composition contains from 5 parts by mass to 20 parts by mass of a plasticizer based on 100 parts by mass of a rubber component.