In a variable compression ratio internal combustion engine in which the
compression ratio is changed by changing the combustion chamber volume,
the present invention enables to inhibit irregularities in the air-fuel
ratio involved by changing of the combustion chamber volume. In a
transition period during which the compression ratio is changed, the
actual volume of the combustion chamber realized by a compression ratio
changing mechanism is detected. A factor that influences the air-fuel
ratio (for example, fuel injection quantity, cylinder intake air quantity
or amount of EGR) is controlled based on the actual volume of the
combustion chamber thus detected to make the air-fuel ratio substantially
equal to that before and after the change of the compression ratio.