PURPOSE: To make the exhausting conditions of exhaust pipes uniform and to utilize effectively the pulsation of exhaust gas so as to improve the output performance, by making the difference of the lengths of the exhaust pipes small and the curvatures thereof large.

CONSTITUTION: Of the two exhaust pipes of a front cylinder row 32, the non- silencer side exhaust pipe 36A is connected to a silencer 56 in such a way that it extends in front of the silencer side exhaust pipe 36B and is positioned upward. Of the two exhaust pipes of a rear cylinder row 34, the non-silencer side exhaust pipe 38A is connected to the silencer 56 in such a way that it is positioned above the silencer side exhaust pipe 38B and is bent by 180° below the engine 30. Thus, the lengths of all the exhaust pipes are made equal, and the curvature radii thereof are made relatively large.