To provide a cord for a transmission belt and the transmission belt, especially capable of maintaining a belt tension even after running for a long period as the frictional transmission belt, and reducing noise or vibration on traveling.

This rubber-reinforcing cord used for the transmission belt, consisting of a polybutylene terephthalate fiber, treated with first twists and ply twists of 300-1,500 dtex//1-3 pieces (S- or Z-twisting direction)/2-5 pieces (Z- or S-direction) and having a both twists structure having 600-2,500 twist factor of the first and ply twists is provided by having 0.03-0.20 cN/dtex heat shrinkage stress and 0.2-1.0 cN/dtex stress at 5% stretch. The transmission belt by using the cord is provided.