To improve durability of a belt and reduce occurrence of noise by the belt by preventing the occurrence of resonance phenomenon of a power transmission belt.

A power transmission belt 20 consists of first, second, and third belts. The back part 26 and the tooth flank part 23 of a first belt part A1 are formed of a first base material. The back face part 26 and the tooth flank part 23 of a second belt part A2 are formed of a second base material having weight per unit length in a belt longitudinal direction lower than that of the first base material. The back face part 26 and the tooth flank part 23 of a third belt part A3 are formed of the first and second base materials. A unit weight is rendered uneven along the peripheral direction of the power transmission belt 20 and resonance phenomenon is prevented from occurring.