In an internal combustion engine, a sulfur detection sample gas is generated in a sample gas generation chamber (14) by fuel of an amount smaller than the amount of fuel fed to a combustion chamber (2) and proportional to the amount of fuel fed to the combustion chamber (2). An SO X sensor (16) 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 SO X that flows into the catalyst (8) is estimated from the output value of this SO X sensor (16).