An exhaust control system for an internal combustion engine, comprising: a NOx reduction filter that is provided in an exhaust passage of the internal combustion engine to trap a particulate matter included in an exhaust gas and is configured by supporting a selective catalytic reduction NOx catalyst that selectively reduces NOx in the exhaust gas using ammonia as a reducing agent; a reducing agent supplier that supplies ammonia or a precursor of ammonia to the NOx reduction filter via the exhaust gas flowing into the NOx reduction filter; a trapping capability acquisition part that is configured to obtain information regarding a trapping capability of the NOx reduction filter to trap the particulate matter in the exhaust gas, based on a detection value of an exhaust sensor provided to detect a predetermined parameter relating to the exhaust gas flowing out of the NOx reduction filter; a NOx reduction capability acquisition part that is configured to obtain information regarding a NOx reduction capability of the NOx reduction filter, based on an amount of NOx in the exhaust gas flowing out of the NOx reduction filter, the exhaust control system being characterized in that it further comprises a determination part that is configured to determine and distinguish between a trapping abnormal state in which a trapping function of the particulate matter by the NOx reduction filter is failed and a sensor abnormal state in which a detection function of the predetermined parameter by the exhaust sensor is failed, based on the information regarding the NOx reduction capability, when the information regarding the trapping capability indicates that the trapping capability is in a predetermined low trapping capability state. a NOx reduction filter that is provided in an exhaust passage of the internal combustion engine to trap a particulate matter included in an exhaust gas and is configured by supporting a selective catalytic reduction NOx catalyst that selectively reduces NOx in the exhaust gas using ammonia as a reducing agent; a reducing agent supplier that supplies ammonia or a precursor of ammonia to the NOx reduction filter via the exhaust gas flowing into the NOx reduction filter; a trapping capability acquisition part that is configured to obtain information regarding a trapping capability of the NOx reduction filter to trap the particulate matter in the exhaust gas, based on a detection value of an exhaust sensor provided to detect a predetermined parameter relating to the exhaust gas flowing out of the NOx reduction filter; a NOx reduction capability acquisition part that is configured to obtain information regarding a NOx reduction capability of the NOx reduction filter, based on an amount of NOx in the exhaust gas flowing out of the NOx reduction filter, the exhaust control system being characterized in that it further comprises a determination part that is configured to determine and distinguish between a trapping abnormal state in which a trapping function of the particulate matter by the NOx reduction filter is failed and a sensor abnormal state in which a detection function of the predetermined parameter by the exhaust sensor is failed, based on the information regarding the NOx reduction capability, when the information regarding the trapping capability indicates that the trapping capability is in a predetermined low trapping capability state.