A concentration of sulfur components contained in fuel is determined, without adding any special sensor for determining the sulfur concentration of the fuel. To this end, a sulfur concentration determination system for an internal combustion engine is provided with an exhaust gas purification device including an oxidation catalyst disposed in an exhaust passage, and a supply device for supplying fuel to exhaust gas flowing into the exhaust gas purification device, wherein after the end of poisoning elimination processing of the oxidation catalyst, fuel supply processing is carried out in which the fuel is supplied to the exhaust gas purification device in such a manner that an amount of fuel supplied to the exhaust gas purification device per unit time becomes constant, and the concentration of sulfur components contained in the fuel is determined by using as a parameter a period of time (an oxidation reaction period of time) required from a starting point in time of the fuel supply processing until the temperature of the oxidation catalyst drops to an initial temperature or below.