To prevent the early crush of the thread of a tension adjusting rod while preventing the early cutting of a hook part.

This belt transmission gear is provided with a tension coil spring 16 for energizing a tension arm 15 for applying tension to a transmission belt 14, to be operated in a tension applying direction; a tension adjusting rod 18 axially changed in position to the tension arm 15 by the thread-fastening operation of a nut 17 to change the tension of the tension coil spring 16; and a locknut 19 for locking the nut 17. A contact preventing fitting part 17a rotatably fitted into a rod through hole of the tension arm 15 to prevent the contact of the tension adjusting rod 18 with the peripheral wall of the rod through hole is provided in line with the nut 17.