PURPOSE: To prevent icing at the time of idling operation, by providing an auxiliary intake pipe independently of a main intake pipe, locating inlet ports of the auxiliary intake pipe at a portion communicated to the atmosphere as well as at a portion opened and closed by a throttle valve, and thereby increasing the air flow rate at the time when the opening of the throttle valve is large.

CONSTITUTION: In a cylinder 5, an auxiliary intake pipe 7 is provided independently of a main intake pipe 2 in the manner that it opens at the back of an intake valve 8. These auxiliary intake pipes are provided commonly for each cylinder 5 and they are extended via a concentrated pipe 9 toward a carburetor 3. The top of the concentrated pipe 9 is branched into inlet ports 9a, 9c, which are opened respectively at a position located downstreams of the outlet of an air cleaner 10 and a position opened and closed by a throttle valve 3a of a carburetor 3. Since, with such an arrangement, fluid is supplied to the back of the intake valve 8 from both of the inlet ports 9a, 9c, it is enabled to control the amount of fluid to be sufficient. Further, at the time of idling operation, the amount of gas to be supplied to pipe 9 is controlled by the throttle valve 3a.