An abnormality detection system of an engine exhaust system in an internal combustion engine in which an NO x storage catalyst able to store NO x when an air-fuel ratio of exhaust gas is lean and able to release stored NO x by making the air-fuel ratio of the exhaust gas rich is arranged in an engine exhaust passage; a hydrocarbon feed valve is arranged in the engine exhaust passage upstream of the NO x storage catalyst; and a particulate filter for trapping particulate matter contained in the exhaust gas is arranged in the engine exhaust passage downstream of the NO x storage catalyst, wherein the hydrocarbon feed valve is configured to inject hydrocarbons to make the air-fuel ratio of the exhaust gas flowing into the NO x storage catalyst rich when NO x should be released from the NO x storage catalyst, characterized by: an electric resistance type sensor having a sensor part to which particulate matter and hydrocarbons which are contained in exhaust gas deposit and configured to generate an output value corresponding to an amount of deposition of the particulate matter and hydrocarbons to said sensor part; wherein: the electric resistance type sensor is arranged in the engine exhaust passage downstream of the particulate filter; the electric resistance type sensor is configured to, when hydrocarbons are injected from the hydrocarbon feed valve to release NO x from the NO x storage catalyst and when hydrocarbons slip through the NO x storage catalyst, have an output value which exhibits a behavior which changes by a faster speed compared with when particulate matter slips through the particulate filter, then changes in direction of change to an opposite direction; and when the output value of the electric resistance type sensor changes, the system is configured to judge if hydrocarbons have slipped through the NO x storage catalyst when hydrocarbons are injected from the hydrocarbon feed valve or particulate matter has slipped through the particulate filter from the difference in behavior of the output value of the electric resistance type sensor. an electric resistance type sensor having a sensor part to which particulate matter and hydrocarbons which are contained in exhaust gas deposit and configured to generate an output value corresponding to an amount of deposition of the particulate matter and hydrocarbons to said sensor part; wherein: the electric resistance type sensor is arranged in the engine exhaust passage downstream of the particulate filter; the electric resistance type sensor is configured to, when hydrocarbons are injected from the hydrocarbon feed valve to release NO x from the NO x storage catalyst and when hydrocarbons slip through the NO x storage catalyst, have an output value which exhibits a behavior which changes by a faster speed compared with when particulate matter slips through the particulate filter, then changes in direction of change to an opposite direction; and when the output value of the electric resistance type sensor changes, the system is configured to judge if hydrocarbons have slipped through the NO x storage catalyst when hydrocarbons are injected from the hydrocarbon feed valve or particulate matter has slipped through the particulate filter from the difference in behavior of the output value of the electric resistance type sensor. the electric resistance type sensor is arranged in the engine exhaust passage downstream of the particulate filter; the electric resistance type sensor is configured to, when hydrocarbons are injected from the hydrocarbon feed valve to release NO x from the NO x storage catalyst and when hydrocarbons slip through the NO x storage catalyst, have an output value which exhibits a behavior which changes by a faster speed compared with when particulate matter slips through the particulate filter, then changes in direction of change to an opposite direction; and when the output value of the electric resistance type sensor changes, the system is configured to judge if hydrocarbons have slipped through the NO x storage catalyst when hydrocarbons are injected from the hydrocarbon feed valve or particulate matter has slipped through the particulate filter from the difference in behavior of the output value of the electric resistance type sensor.