An exhaust gas purifying system for an internal combustion engine has a relatively simple construction and is capable of accurately calculating the amount of SOx deposited on a NOx catalyst.A CO2 sensor having a detection electrode containing BaCO3 is disposed in an intake pipe at a location downstream of a confluent portion. A catalyst SOx deposition amount is calculated according to an EGR rate and a sensor deposition amount, based on the fact that the relationship between the sensor deposition amount detected by the CO2 sensor and the catalyst SOx deposition amount is determined according to the EGR rate.