Abstract: The present invention is provided with an NOx catalyst 17 provided in the exhaust passage of an internal combustion engine capable of lean burn and adapted to release and reduce absorbed NOx by a reducing agent, a reducing agent supplying means 19 provided in the exhaust passage on the upstream side of the NOx catalyst 17, a load detecting means for detecting the load of an internal combustion engine 1, and a reducing agent addition control means for controlling the reducing agent addition period and the reducing agent addition interval on the basis of the load of the internal combustion engine 1. The addition period and the addition interval for the reducing agent are controlled taking into account, for example, the degree to which reducing agent adheres to the wall surface in the exhaust passage depending on the velocity, temperature, etc.