PURPOSE: To increase noise elimination effect by improving the way of exhausting the exhaust gas through an exhaust pipe, an expansion chamber, a noise elimination underwater chamber, and an auxiliary exhaust gas outlet.

CONSTITUTION: An auxiliary exhaust pipe 31 connected to a connecting hole 32 in a main exhaust pipe 23 is provided between the main exhaust pipe 23 and an expansion chamber 20. The auxiliary exhaust pipe 31 is provided with a noise eliminating function to eliminate noise before exhaust gas is led to the expansion chamber 20.