A NOx catalyst for selectively reducing NOx in exhaust gas by adsorbing ammonia is provided at an exhaust system of an engine, ammonia or urea water is supplied to the NOx catalyst by reducing agent supplying means, a consumption amount of ammonia adsorbed to the NOx catalyst is derived by consumption amount deriving means based on an exhaust amount of NOx exhausted from the engine detected or estimated by NOx exhaust amount deriving means and an actual NOx cleaning rate by the NOx catalyst derived by actual NOx cleaning rate deriving means, an actual adsorption amount of ammonia adsorbed to the NOx catalyst is derived in accordance with an addition amount of ammonia and the consumption amount of ammonia by adsorption amount deriving means and the reducing agent supplying means is controlled by controlling means in accordance with the actual adsorption amount.