To uniformly disperse reducing agents in exhaust gas by providing swirl generator for providing a swirl to an exhaust gas flow in an exhaust pipe upstream from a supply pipe in a device having an NOx catalyst in the exhaust pipe and the supply pipe for introducing reducing agents in the upstream thereof.

Exhaust gas from a diesel engine is passed through an exhaust pipe, cleaned by an NOx catalyst and then discharged into the atmosphere from a muffler. At this time, a pump 4 in a reducing agent supply pipe 10 is actuated, and the proper amount of hydrocarbon reducing agents in a reducing agent tank is injected and added from the supply pipe 10 to the downstream side of a swirl generator 9 provided in the exhaust pipe in the upstream position of the NOx catalyst at a proper time. This swirl generator 9 is constructed in such a manner that a plurality of arcwise laminas 22 are extended radially from the vicinity of the center through-hole 21 of a disk 20 in the radial direction of the disk 20. Thus, the reducing agents injected from the tip part of the supply pipe 10 are sprayed in mists by exhaust gas passed through the center through-hole 21, and then uniformly dispersed in the exhaust gas.