PURPOSE: To prevent the thermal deterioration caused by the excessive temperature rise of the catalyst without using a catalyst temperature sensor or even when the sensor is failured in an exhaust emission control system where a bypass passage to bypass an NOx purifying catalyst is provided and the exhaust gas flows in the bypass passage when the temperature of the catalyst is high.

CONSTITUTION: The running condition of an engine is detected by an engine load sensor 6 and an engine speed sensor 7, a judgement is made on the shifting tendency of the temperature of the catalyst 1 based on the running condition of the engine, and the opening/closing of a bypass valve 5 is controlled by a valve controlling means 10 based on the results of the judgement.