A belt transmission system comprises: a first pulley; a second pulley; a belt; and an automatic tensioner having a first tension pulley and a second tension pulley applying tension to a portion of the belt located at a tension side and a portion of the belt located at a slack side of the belt, and an arm. Force from the belt makes the first tension pulley and the second tension pulley rotate in the same direction. When the first tension pulley rotates in a direction tensioning the belt, the second tension pulley also rotates in a direction tensioning the belt.