PURPOSE: To prevent fuel from leaking from a throw passage when an engine is started, or when compression pressure is measured.

CONSTITUTION: The outlet end of an opening degree detecting switch 19 arranged on a throttle valve 18 is connected to a controller 12 for controlling an idling solenoid valve 11 provided in the throw passage 10 of a carburetor. In condition in which fuel in a cylinder becomes overflow and it is difficult to be started an engine, when the throttle valve 18 is in a set opening degree or more, the idling solenoid valve 11 is operated by the controller 12, and the throw passage 10 is closed so as to enable the engine to be started easily. And also fuel does not flow out to an exhaust passage since the throw passage 10 is shut out in the same way when compression pressure is measured in an engine warming up condition, so that it is possible to protect a catalyst arranged in an exhaust passage.