An exhaust purification system for an internal combustion engine is provided that can maintain the NOx purification rate of a selective reduction catalyst at near the maximum thereof. The exhaust purification system is provided with an oxidation catalyst and CSF provided in the exhaust plumbing of the engine, a selective reduction catalyst that is provided in the exhaust plumbing on the downstream side of oxidation catalyst and CSF, and selectively reduces NOx in the exhaust, and a NO2 sensor that detects NO2 in the exhaust inside of the exhaust plumbing on the downstream side of the selective reduction catalyst. An ECU executes NO2-NOx ratio decrease processing to cause the NO2-NOx ratio corresponding to the ratio of NO2 to NOx in the exhaust flowing into the selective reduction catalyst to decrease, in a case of a detection value Vno2 from the NO2 sensor being greater than a predetermined value Vno2_th.