An impeller for pressurizing an intake air of an engine is provided on a left side end portion in a supercharger rotary shaft of the supercharger. The power from the engine is transmitted to a second sprocket, which is provided on a right side end portion of an input shaft of a speed increaser, through a chain to thereby allow the supercharger to drive. The second sprocket is removably fitted to the input shaft by means of a bolt. The supercharger rotary shaft, the input shaft, the second sprocket and the bolt are all accommodated in a supercharger casing. The supercharger casing has a right side end face formed with an access opening through which access can be made to the second sprocket and the bolt from an axial direction.