Emission control apparatus of internal combustion engine

A NOx occluding member (23) that occludes NOx when the air-fuel ratio is on the fuel-lean side is disposed in an engine exhaust passage. An NOx ammonia sensor (29) is disposed in the engine exhaust passage downstream of the NOx occluding member (23). A surplus amount of a reducing agent that is not used to release NOx is determined from a change in the ammonia concentration detected by the NOx ammonia sensor (29) when the air-fuel ratio is changed to the fuel-rich side so as to release the NOx from the NOx occluding member (23).