An auto-tensioner for a belt has a cylindrical body having a bottom. A nut formed with female threads on its inner periphery and having a rod integrally formed at one end is slidably mounted in the body. A bolt is mounted in the body so that its male threads will engage the female threads on the nut. A belt loading spring is mounted between the nut and the bottom of the body to urge the nut in such a direction as to increase the tension to the belt. A bolt turning spring is mounted between the nut and the bolt to urge and turn the bolt in such a direction as to be pressed against the bottom of the body or in a direction away from the nut. Thus the nut and the bolt are moved either toward or away from each other according to the load from the belt which acts on the rod so as to automatically adjust the tension to the belt.