PURPOSE: To perform timing control for the arrival of return acoustic pressure waves and the reduction of pollutants by providing an exhaust gas pulse passage which is closed except in communication with a first exhaust gas opening and an exhaust gas discharge passage independent from the exhaust gas pulse passage in an exhaust gas housing.

CONSTITUTION: An exhaust gas discharge passage 303 extends downwardly from the upper end of an exhaust housing 91, communicates with an exhaust passage opening 167 in the lower face 41 of an engine block and extends backwardly to an exhaust gas pulse passage 301 for exhausting air through exhaust ducts for a driving shaft housing and a gear case at the lower end. The control operation of an exhaust gas device 111 executed by the exhaust gas pulse passages 131, 301 is separated from the exhaust gas discharge operation provided by an exhaust passage 133 and an exhaust passage 303 so that acoustic pressure waves from the housing is not attenuated by passing through a catalyst element 177. Exhaust gas discharge operation and control operation are kept separated in an engine block and in an exhaust housing 91.