In a fuel injection control method of an engine which an intake air flow rate is estimated by a predetermined calculating method, using data on detected intake pipe pressure and engine rotating speed, a fuel injection time is determined by a predetermined calculating method, using the intake air flow rate, and a deviation from a target engine rotating speed, and an injector driving signal is outputted by a fuel injection control device, the fuel injection time is obtained by adding a battery voltage fluctuation correction time, and a transient correction injection time which corrects a deviation from the target engine rotating speed, which is obtained by a predetermined calculating method, to a basic injection time obtained by a predetermined calculating method.