PURPOSE: To reduce fuel consumption, by successively opening and closing throttle valves in each carburetor correspondingly with a phase difference to a throttle control amount, fully closing at least one of the throttle valves at low load operation and blocking a circulative flow of intake air.

CONSTITUTION: A device is constituted such that throttle valves 22A, 22B, 22C are provided in each carburetor 18AWC, while connected to a throttle wire 30 through interlocking mechanisms 24A, 24B, and successively opened and closed. While solenoid valves 62B, 62C, closing a fuel supply passage of low speed system at fully closing of the throttle valves 22B, 22C, are provided in the carburetors 18B, 18C. If the throttle wire 30 is returned to a maximum, the throttle valves 22B, 22C are placed to a fully closed position to block a flow of intake to cylinders 12B, 12C, and only the throttle valve 22A is opened to operate only a cylinder 12A. Further the fuel supply passage of low speed system is closed, thus fuel is never consumed uselessly at low load operation.