A straight-flow exhaust purification catalyst (13) and a hydrocarbon supply valve (15) are arranged in the engine exhaust passage of an internal combustion engine. A region which is a limited portion of the upstream-side end-surface peripheral part of the exhaust gas purification catalyst (13), where there is a possibility of clogging due to deposition of fine particles in the exhaust gas, is predicted to be a fine particle deposition region, and an air-fuel ratio sensor (23) is arranged within an exhaust gas circulation region, which is downstream from the downstream-side end-surface peripheral part of the exhaust purification catalyst (13) and is downstream from the fine particle deposition region when viewed along the longitudinal axis of the exhaust purification catalyst. When the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst (13) momentarily changes and the rate of change of the output value from the air-fuel ratio sensor (23) decreases, it is determined that clogging due to deposition of fine particles in the exhaust gas has occurred in the fine particle deposition region in the upstream-side end-surface peripheral pat of the exhaust purification catalyst (13).