PURPOSE: To stabilize idle operation by obtaining each coefficient related to each opening area of a throttle valve and a control valve in an auxiliary air passage when idle operation condition is fulfilled and calculating fuel amount from the above-described values.

CONSTITUTION: In order to obtain the fundamental injection time Ti of a fuel injection valve 12 when the idle operation condition is fulfilled, the opening area coefficient value Kθ of a throttle valve is obtained according to the opening degree of the throttle valve which is obtained by a sensor 17. The opening area coefficient value KAIC of the first control valve 6 installed in an auxiliary air passage is obtained according to the opening area of the first control valve, namely the valve opening duty ratio, and the opening area coefficient KFI is obtained according to the cooling-water temp. detected by a cooling-water temp. sensor 13, and the opening area coefficients KAC and KAT are obtained according to the ON/OFF state of an air conditioner and the engagement state of an automatic speed change gear respectively. The necessary fundamental injection time Ti is determined by multiplying the total sum of the above-described coefficients by the pulse generation time intervals of the top dead center signals.