Emission control system for internal combustion engine

An exhaust gas purification system is composed of a catalyst for selective catalytic NOx reduction (NOx catalyst) is arranged on an exhaust passage and an oxidation catalyst disposed upstream thereof, in which a fuel poisoned state of the NOx catalyst preferably dissolved in connection with a fuel supply to the exhaust gas. In the exhaust gas purifying system, the NOx catalyst is designed so that a pore diameter of zeolite having thereof a size such that NOx and ammonia can pass, however, a predetermined hydrocarbon can not pass; and the NOx catalyst has a property such that the fuel contained in the exhaust gas is modified to a fuel with lower molecular weights and is enlarged, the pore diameter of zeolite in the formation of the NOx catalyst, when the catalyst temperature thereof is within a predetermined high-temperature region. Then, for an increase of a temperature of the NOx catalyst to a predetermined target temperature is carried out a temperature increasing operation to dissolve the fuel poisoning state in which a fuel supply is carried out from an upstream side of the oxidation catalyst by means of a fuel supply unit according to a fuel supply condition under which the fuel supplied not from the downstream side of the oxidation catalyst is discharged, when it is judged that the pore inner portions of the NOx catalyst are located in the fuel poisoned state.