Abstract: A belt tensioner has a shaft having an external thread and rotatably mounted in a body, a torsion spring provided between the body and the shaft so as to urge the shaft in a rotational direction, and a cylindrical rod having an internal thread engaged with the external thread of the shaft. A lower end of the rod is projected from the body so as to be operatively connected to a tension pulley for a belt. The torsion spring and both the threads are arranged to project the rod by the rotation of the shaft urged by the torsion spring. The rod is secured to the body so as to be adjusted in angular disposition.