To activate an exhaust emission control device by increasing the temperature of exhaust gas from an diesel engine (DE) during low-speed operation, trapping the exhaust gas in a high-temperature condition and feeding it in such a condition to an DE auxiliary exhaust emission control device.

The exhaust emission control device is provided for the multicylinder DE having an inlet throttle valve and an exhaust recirculating exhaust auxiliary cam. It is first characterized in that exhaust gas is fed through a main exhaust passage to a main exhaust emission control device and an exhaust inlet pipe is provided across a branch pipe of an exhaust manifold in communication therewith and connected to the auxiliary exhaust emission control device and further to the main exhaust passage via a selector valve so that the inlet throttle valve, the exhaust auxiliary cam and the selector valve are operated by an actuator to be operated with a control device signal responsive to the operated condition of the DE. It is secondly characterized in that the auxiliary exhaust inlet pipe has an end inserted into an exhaust port and a base end connected to the exhaust inlet pipe. The temperature of the exhaust gas from the DE during, particularly, low-speed operation is increased and the exhaust gas is trapped in a high-temperature condition and fed in such a condition to the DE auxiliary exhaust emission control device, resulting in the activated exhaust emission control device with active purifying operation.