To optimize an air/fuel ratio at starting, assure startability and improve exhaust emission by complementing the lower detecting precision of an air flow sensor at engine start.

The engine 1 is provided with an intake pressure sensor 15 for detecting pressure in an intake pipe 9 into which a fuel injection valve 16 injects a fuel and the air flow sensor 12 for detecting the flow rate of air in the intake pipe 9. After starting the engine but before a preset time passes, a fuel injection amount is set in accordance with an intake air amount computed from a value detected by the intake pressure sensor 15, and after the preset time passes, the fuel injection amount is set in accordance with an intake air amount computed from a value detected by the air flow sensor 12.