PURPOSE: To improve the accelerating performance, in a system where a starter valve is opened and a rich fuel is fed at start of engine, by opening the starter valve when the opening of a throttle valve is wide while the intake negative pressure is low, thereby preventing leaning of air-fuel ratio.

CONSTITUTION: A starter outlet 3 is opened to an intake path 2 provided with a throttle valve 1 and fed with mixture of starter gas 4 and rich fuel fed through a starter nozzle 5. The starter nozzle 5 is opened/closed by means of a starter valve 6 slidable freely up and down where said valve 6 is constituted into a solenoid valve 9 containing an electromagnetic coil 8. The starter valve 6 is opened manually when starting an engine under low temperature so that the starting is facilitated while current is applied through a throttle switch 10 closable upon opening of the throttle valve 1 over the predetermined opening and a booster switch 11 closable upon droppage of the intake negative pressure below the predetermined opening, thereby it is possible to improve the acceleration performance.