In an internal combustion engine, a sulfur detection sample gas is generated in a sample gas generation chamber by fuel of an amount smaller than the amount of fuel fed to a combustion chamber and proportional to the amount of fuel fed to the combustion chamber. An SOx sensor having a sensor part trapping the sulfur contained in the sample gas and capable of detecting the amount of sulfur trapped in the sensor part from the property changes of the sensor part is provided. The amount of SOx that flows into the catalyst is estimated from the output value-of this SOx sensor.