This internal combustion engine is provided with a variable compression ratio mechanism capable of changing a mechanical compression ratio by changing the volume of the combustion chamber at the top dead center. The pressure and temperature of remaining combusted gas within the combustion chamber at the time when the exhaust valve is closed in the intake stroke is measured or estimated, the pressure and temperature of intake air supplied into the combustion chamber after the exhaust valve is closed in the intake stroke is measured or estimated, and based on the assumption that the pressure and temperature of the remaining combusted gas which saturates the volume of the combustion chamber at the time when the exhaust valve is closed in the air intake stroke become, when the intake air is supplied to the combustion chamber, equal to the pressure and temperature of the intake air, the volume of the remaining combusted gas after the change is calculated.