The internal combustion engine has an ammonia feeder which feeds ammonia to a combustion chamber and an NOX selective reduction catalyst which is arranged in an engine exhaust passage. The removal rate of the NOX selective reduction catalyst depends on a ratio of concentration of ammonia to NOX of the exhaust which flows into the NOX selective reduction catalyst, that is, a ratio of concentration of inflow. A high removal rate range where the NOX removal rate in the NOX selective reduction catalyst becomes substantially maximum and, furthermore, the ammonia removal rate becomes substantially maximum, is set in advance. The internal combustion engine is controlled so that the ratio of concentration of inflow becomes inside the high removal rate range.