PROBLEM TO BE SOLVED: To provide a power transmission belt of a unitary construction having high rigidity, durability and flexibility.SOLUTION: The power transmission belt is formed of a belt-shaped annular element having predetermined rigidity and elasticity and is wound around a pair of pulleys for belt transmission between the pair of pulleys. The power transmission belt is provided with outer slits 6 which are opened toward an outer peripheral side in a radial direction of the annular element 1 and have grooves cut from the outer peripheral side toward an inner peripheral side in the radial direction throughout a width direction of the annular element 1, and inner slits 7 which are opened toward the inner peripheral side and have grooves cut from the inner peripheral side toward the outer peripheral side throughout the width direction. The outer slits 6 and the inner slits 7 are formed so that they are situated alternately at a predetermined pitch p in a circumferential direction of the annular element 1 and a sum of the groove depth deo of the outer slits 6 and the groove depth dei of the inner slits 7 is larger than the thickness th of the annular element 1 in the radial direction.