To shift a clarification temp. window for NOx to a high temp. side.

This catalyst for clarification of exhaust gas comprising a carrier comprising of a porous oxide contg. a rare earth element and a catalytic precious metal carried on the carrier is heat-treated with an oxidative atmosphere contg. an acidic gas. It is estimated that the carrier or the catalytic precious metal is changed in properties by this heat treatment and the clarification temp. window for NOx is shifted to the high temp. side.