PURPOSE: To make it possible to precisely control fuel even with no opening degree signal from a throttle valve, by compensating the supply amount of fuel in accordance a bypass air amount signal and a rotational speed signal during engine idle operation.

CONSTITUTION: When a load is exerted to an engine during engine idle operation, a load switch 11 is turned on while a throttle opening degree sensor 2 delivers an idle opening degree signal. A control unit 7 delivers a drive signal to a solenoid valve 4 to control the position of a valve 4a so that bypass air is fed through an outlet port 4c. In synchronization with this step, the control unit 7 obtains an idle load increment fuel injection signal in accordance with a previous set drive current for the solenoid valve, that is, in accordance with the amount of bypass air, from a map, and controls a fuel injection valve 1 in accordance with the signal to inject a compensated amount of fuel corresponding to the increment of the bypass air amount. Accordingly, it is possible to control the compensation of fuel in accordance with the load condition of the engine during idle operation.