PURPOSE: To decrease the discharging amount of unburnt HC by stopping supply of assist air from an assist air supplying port until fuel infection is started from a fuel injection valve in cranking of an internal combustion engine, and starting supply of assist air when fuel injection is started.

CONSTITUTION: In cranking of an engine, supply of assist air from a supplying port 8 for assist air is stopped until fuel injection is started from a fuel injection valve 7, and supply of air from a bypass air supplying port 14a is also stopped. When the fuel injection valve 7 starts fuel injection, the assist air supplying port 8 starts supply of assist air, and the bypass air supplying port 14a starts supply of air. Therefore, fuel having the amount proportional to the intake air amount is injected by the fuel injection valve 7, and the amount of injection fuel can be decreased. Since assist air to be supplied from the assist air supplying port 8 promotes atomization of injection fuel, the discharge of unburnt HC is drastically decreased.