To improve output by changing the shapes of jointed pipes on the way of an exhaust pipe.

A straight pipe A and jointed pipes 6a, 6b constituted by a diffuser B are provided on the rearside of the gathered pipes 5a, 5b of an exhaust pipe or between them and a gathered pipe 7, and the diffuser angle θof the jointed pipe is set between 8°∼10° and the straight pipe for constituting the jointed pipe is set to the length of 35%∼45% of the jointed pipe length.