PURPOSE: To shorten various tests for improving the performance of a fully- opened low-speed operation region and improve the product development efficiency by operating a driving actuator at the time of the fully-opened low-speed operation of an engine, and controlling an intake passage at the area smaller than the effective opening area of a throttle valve.

CONSTITUTION: Detected signal currents outputted from a throttle valve opening detector 20 and a rotating speed detector 31 are inputted to a control unit 30 at the time of the fully-opened low-speed operation of an engine, a driving actuator 24 is operated by the control current outputted from the control unit 30, and an intake passage 2 is controlled at the area smaller than the effective opening area of a throttle valve 7 by an air valve 21. The reduction of the intake negative pressure applied to a needle jet 10 opened at a venturi section V is suppressed even at the time of the fully-opened low-speed operation, a mixture can be prevented from becoming lean, and various tests for improving the performance of a fully-opened low-speed operation region can be sharply shortened.