To facilitate a tension adjusting operation in the chain tension structure without changing a wheelbase of a vehicle. In the chain tension structure in which a chain is wound around a drive sprocket wheel which is arranged below a vehicle body frame and a driven sprocket wheel which is mounted on a rear wheel side and tension is imparted to the chain by a tension roller, the tension roller is mounted in a state that a hexagon socket head cap screw which constitutes a rotary shaft of the tension roller is inserted into an elongated hole which is opened in a plate-like projecting portion formed on the vehicle body frame.