A toothed belt includes: a back portion; and a plurality of helical teeth provided on an inner periphery of the back portion and disposed at a fixed pitch in a belt longitudinal direction. An angle θ between a tooth trace direction of the helical teeth and a belt lateral direction is in a range of 7 degrees to 10 degrees, both inclusive. A ratio A of a thickness tb of the back portion to a tooth height hb of the helical teeth, which is given by A=100×tb/hb, is in a range of 120% to 240%, both inclusive.