To prevent generation of torque steps when A/F ratio is changed, by setting a target intake amount taking into consideration the difference in fuel consumption rate according to the difference in A/F ratio when the A/F ratio setting is changed according to driving conditions, and by controlling a throttle valve opening based on the target intake amount.

To a control unit 16, detection signals from a throttle opening sensor 7, an oxygen sensor 9, an accelerator opening sensor 12, an air flow meter 13, crank angle sensor 14, and a water temperature sensor 15, which detect operating condition of an engine 1 are input, and predetermined control signals are output to a driving unit 6 for a throttle valve 5, a fuel injection valve 2, an ignition plug 3, and an EGR valve 11. A reference intake amount is calculated by summing up an intake amount corresponding to accelerator operation amount and an intake amount for stabilizing idle speed. After a target intake amount is calculated by correcting the reference amount with an intake amount correction value, an opening of a throttle valve 5 is controlled to be a target opening corresponding to the target intake amount.