PURPOSE: To prevent maneuverability from degradation accompanying drop of air fuel ratio produced by connection of a primary throttle valve with a secondary one by varying timing and amount of fuel correction in response to the opening of the secondary throttle valve and the temperature condition of an engine.

CONSTITUTION: A detector 6 for detecting the opening of throttle valve is mounted on a secondary throttle valve 5, and an electromagnetic valve 7 is provided in a path opening to the side wall of a primary air path 2 downstream of a primary throttle valve 4. The path provided with the electromagnetic valve 7 communicates to the downward side of a fuel path 8. The opening of the throttle valve 5 is detected by the detector 6 and the temperature condition of an engine is detected by a water temperature detector 13 to send signals to the input of a control circuit 12. The control circuit 12 sends the output result of calculation to the electromagnetic valve 7 in response to the opening of the secondary throttle valve 5 and the change in temperature of the engine. The electromagnetic valve 7 receives this signal to supply from a fuel path 8 to the engine fuel pressurized at a certain level for a predetermined time.